Fluctuations in the cod stocks of the Gulf of St Lawrence

Transcription

1 ICES mar. Sei. Symp., 198: Fluctuations in the cod stocks of the Gulf of St Lawrence Ghislain A. Chouinard and Alain Fréchet Chouinard, G. A., and Fréchet, A Fluctuations in the cod stocks of the Gulf of St Lawrence - ICES mar. Sei. Symp., 198: The Gulf of St Lawrence is a semi-enclosed sea connected to the Northwest Atlantic Ocean by the Strait of Belle Isle and Cabot Strait. Two Atlantic cod stocks (Gadus morhua) inhabit the area; the southern (NAFO 4T-Vn (Jan.-Apr.)) and northern (NAFO 3Pn,4RS) Gulf of St Lawrence stocks. Catches since 1960 have averaged t and t, respectively. In recent years about 80% of the landings were made by otter trawls and seines. For both stocks, abundance was low in the mid-1970s and in recent years, and was highest in the early 1980s. Landings, abundance, and recruitment trends between the two stocks are relatively similar. Production for both stocks has been low in recent years. Although fishing mortality has fluctuated over the time periods examined, abundance changes appear to be related more to recruitment fluctuations. An index of survival calculated as the ratio of recruitment numbers to parental biomass indicates that, for both stocks, lower levels of spawning biomass in the 1970s produced good recruitment, while the number of recruits produced from the large spawning biomass of the early 1980s was low. Although the lower index of survival in the 1980s corresponds with a period of colder conditions (greater ice extent, lower water temperatures), there is little correlation when the entire time period is examined. The index of survival was weakly correlated with freshwater discharge in the southern Gulf. Until factors influencing survival are more clearly understood, direct estimates of pre-recruit abundance are likely to be more helpful in predicting short-term stock trends. Le Golfe du St Laurent est une mer semi-fermée, reliée à l Atlantique nord-ouest par le Détroit de Belle-Isle et le Détroit de Cabot. Deux stocks de morue (Gadus morhua) habitent cette zone: le stock du sud du Golfe du St Laurent (OPANO 4T-Vn (jan-avr)) ainsi que celui du nord (OPANO 3Pn,4RS). Les prises moyennes depuis 1960 sur ces stocks ont été de et de respectivement. Au cours des dernières années, plus de 80% des débarquements ont été réalisés par les chalutiers et les senneurs. Pour les deux stocks, l abondance était basse dans le milieu des années 1970 et au cours des dernières années. Les stocks étaient des plus abondants au début des années Les tendances des débarquements, de l abondance et du recrutement sont relativement similaires entre les deux stocks. La production des deux stocks a été basse au cours des dernières années. Malgré les fluctuations dans le recrutement, les variations de l abondance semblent être reliées principalement aux fluctuations du recrutement. Un indice de survie (rapport recrutement-biomasse parentale) indique que pour les deux stocks, des niveaux moins élevés de biomasse parentale ont donné lieu à un bon recrutement. Le nombre de recrues produites de la biomasse parentale élevée du début des années 1980 a été faible. Malgré que l indice de survie faible des années 1980 correspond à une période plus froide (plus grande étendue de glace, température de l'eau plus froide), il n existe pas de corrélation lorsqu on examine les données sur la période entière. L'indice de survie était cependant faiblement relié à la décharge en eau douce dans le sud du Golfe. Tant que les facteurs influençant la survie ne seront pas mieux compris, des estimations directes de l abondance des pré-recrues seront plus utiles pour prédire les trajectoires de ces stocks à court terme. Ghislain A. Chouinard: Department o f Fisheries and Oceans, Gulf Fisheries Centre, PO Box 5030, Moncton, NB, Canada E1C 9B6; Alain Fréchet: Department o f Fisheries and Oceans, Institut Maurice-Lamontagne, PO Box 1000, Mont-Joli, Québec, Canada G5H3Z4.

2 122 G. A. Chouinard and A. Fréchet ICES mar. Sei. Symp.. 198(1994) Quebec 4S Newfoundland 4T New Brunswick P.E.I 4Vn 3Ps Nova Scotii 4W 4Vs 4X Figure 1. Map of the Gulf of St Lawrence showing some of the physical features and the Northwest Atlantic Fisheries Organization (NAFO) Divisions in the area. Introduction Located on the eastern coast of Canada, between 45 and 52 N, the Gulf of St Lawrence (Fig. 1) is a marginal sea of the Northwestern Atlantic. This area is divided into three Divisions of the Northwest Atlantic Fisheries Organization (NAFO): 4R and 4S in the north and4t in the south. The area of Cabot Strait at the southern entrance to the Gulf of St Lawrence forms NAFO unit areas 3Pn and 4Vn. The cod fisheries of the Gulf of St Lawrence exploit two different stocks of Gadus m orhua: a northern stock and a southern stock, commonly referred to as the 3Pn, 4RS and the 4T-Vn (Jan.-April) cod stocks, respectively. Both stocks undergo an extensive annual migration (Halliday and Pinhorn, 1982). In summer, the southern stock is found in Division 4T while the northern stock is found in Divisions 4R and 4S. During the late fall, both stocks migrate outside the Gulf of St Lawrence to Cabot Strait, where they overwinter. It is thought that mixing of the water in the fall generates very cold waters from the surface down to over 150 m which contributes to the displacement of cod to deeper waters in winter (Fréchet, 1990). The northern Gulf cod stock moves to area 3Pn, while the southern stock migrates to 4Vn. The migrations are thought to extend farther south (areas 3Ps and 4Vs) from time to time. In the spring, as ice drifts out of the Gulf of St Lawrence, the two stocks return to their spawning and feeding grounds in the Gulf. The northern and southern Gulf cod stocks are relatively well segregated. Numerous tagging studies show little mixing between the two (McCracken, 1958; M artin, 1962; Templeman, 1979). More recently, a study of 17 sites in the northern part of the Gulf showed that, with the exception of one site at 10%, less than 3% of the recaptures were made within the boundaries of the southern stock (Gascon et ai., 1990). Both stocks are slow-growing and late-maturing compared to others, such as the North Sea or the Georges Bank stocks.

3 ICES mar. Sei. Sytnp., 198(1994) Fluctuations in the cod stocks o f the G u lf o f St Lawrence 123 The Gulf of St Lawrence has a surface of approximately km2, which makes it about the same size as the Persian Gulf or about 2.5 times smaller than the North Sea. The Gulf of St Lawrence is connected to the Atlantic Ocean by Cabot Strait to the south and by the Strait of Belle Isle to the north. It is bisected by the Laurentian Channel. The area south of the Laurentian Channel is relatively shallow and flat with depths seldom exceeding 75 m, while the area to the north is typified by two deep channels (Anticosti and Esquiman) with depths of 200 to 500 m. In the central southern Gulf and along the north coast in the northern Gulf, bedrock is at or near the surface (Loring and Nota, 1973). One of the characteristics of the Gulf of St Lawrence is that it receives over half of its total freshwater input ( 600 k m per year) from a single source; the St Lawrence River. This large discharge is a major driving element of water circulation in the area (Strain, 1988), particularly over the southern Gulf. The deeper waters of the Laurentian Channel and the northern Gulf are influenced by an inflow of the inshore branch of the Labrador Current through the Belle Isle and Cabot straits. The area is subject to large annual variations in temperature. The Gulf of St Lawrence is covered by ice in winter. Ice formation starts in December-January, reaches its largest coverage by March, then recedes and has usually disappeared by May (June in the northernmost areas). As the ice cover expands from the northwest to the southeast, considerable mixing (upwelling, jets, etc.) occurs at the ice edge (Fréchet, 1990). A permanent feature of the Gulf of St Lawrence is a mass of cold water (<0 C) between 70 and 90 m. During summer, surface waters can reach C. Salinity in the area ranges from about 27 to 35 ppm. In terms of economic returns and employment, the crustacean fisheries [lobster (H om arus americanus), snow crab (Chionoecetes opilio), and shrimp (Pandalus borealis)] are the most important in the Gulf of St Lawrence. However, in terms of landings, the cod fisheries of the Gulf of St Lawrence are the most significant, averaging t per year since Other commercial marine fisheries of importance include herring (Clupea harengus), redfish (Sebastes spp.) and various flatfish species. Chadwick and Sinclair (1991) concluded that fisheries production in the Gulf of St Lawrence was comparable to other coastal areas of similar size. This article describes the cod fisheries in the Gulf of St Lawrence, their trends, the fluctuations in the abundance of the stocks that support them, and the variation in the environment to which they are exposed. Causes of fluctuations in these stocks in the last 20 years and the relationship to the environment are examined. The cod fisheries of the Gulf of St Lawrence The two cod stocks have been exploited by man, to some extent at least, since the 16th century voyage of the French explorer Jacques Cartier to the Gulf of St Lawrence. During that century, French fishermen regularly embarked on fishing expeditions to the Gulf of St Lawrence and adjacent areas (de la Morandière, 1962; de la Villemarqué, 1990). In this century, prior to the mid- 1950s and early 1960s, landings statistics for these two cod stocks are less reliable because landings were reported by port of landing as opposed to area fished. Given the migratory nature of these stocks, the computation of these landings statistics requires that assumptions be made as to the fishing grounds. With the creation of the International Commission for the N orthwest Atlantic Fisheries (ICNAF), the precursor of N AFO, in the early 1950s, and the progressive improvement in the understanding of the migrations of these stocks, reporting of landings statistics was refined for the needs of tracking abundance of stocks (Halliday and Pinhorn, 1990). As a result, reliable landing statistics for the northern stock are only available from about 1960 (Fréchet and Gagnon, 1993). Landings statistics for the southern Gulf stock were obtained from annual fisheries statistics bulletins for the period (Anon., ) and from Chouinard et al. (1992) and Sinclair (1993) since then. Statistics for the period do not include French catches; however, there are no indications that these may have been substantial. Although otter trawlers and other types of mobile gears were being used widely in the waters of the East Atlantic, the cod fisheries exploiting the Gulf of St Lawrence stocks were mostly being conducted by hookand-line between 1920 and the late 1940s. This was due to a limit on the number of trawlers imposed when fixedgear fishermen voiced concerns over the effect of trawling on fisheries resources (Parsons, 1993). As for most other demersal fisheries in the Northwest Atlantic, management was largely done by setting minimum mesh size regulations until 1974, when ICNAF introduced total allowable catches (TACs). The general decline in groundfish resources prompted the declaration of extended jurisdiction by Canada in 1977 and the exclusion of all non-canadian fleets from the fishery for these stocks, with the exception of France. In recent years, the management schemes for these resources have become increasingly complex, with the TAC divided between the various fleets based on historical performance. A system of enterprise allocations [also called individual transferable quota (ITQ)] was implemented for some fleets in 1983 in the northern Gulf and in 1988 for the southern Gulf. Late in 1992, fleets

4 124 G. A. Chouinard and A. Fréchet ICES mar. Sei. Symp (1994) were required to submit conservation plans before they were allowed to fish. Northern Gulf of St Lawrence A description of the fishery in the 1950s in N AFO 4R and 4S is given in Wiles and May (1968). In the early 1960s, landings ranged up to t, then fluctuated around t per year (Fig. 2). Landings peaked at t in 1970, then progressively declined to t in Subsequently, landings increased to attain the high levels of the early 1960s in the period Since then, landings have steadily declined and have been about t in 1991 and Until 1977, the stock was exploited by Canada, France (metropolitan and Saint Pierre and Miquelon), Portugal, and Spain; only France was allowed to continue after extended jurisdiction. The stock is exploited by all adjacent Canadian provinces (Québec, the Maritime provinces and Newfoundland). Prior to 1977, approximately half of the landings were made by non-canadian fleets; the proportion dropped and remained at less than 15% subsequently. Two main gear sectors harvest the northern cod stock: the fixed (gillnets, long-lines, traps and hand-lines) and the mobile (otter trawls and seines). Landings from the (0 a> c c o (a) Northern Gulf Landings Total Allowable Catch (b) Southern Gulf (0 a> c o Landings Total Allowable Catch Figure 2. Landings and total allowable catch (TAC) for (a) the northern and (b) southern Gulf of St Lawrence cod stocks.

5 ICES mar. Sei. Symp., 198 (1994) Fluctuations in the cod stocks o f the G ulf o f St Lawrence 125 w a> c c o (a) Northern Gulf Mobile gears... Fixed gears ) c o (b) Southern Gulf Mobile gears Fixed gears Figure 3. Landings of cod by gear type for (a) the northern and (b) southern cod stocks of the Gulf of St Lawrence. Mobile gears include otter trawls and seines, fixed gears include gillnets, hand-lines, long-lines, and traps. fixed-gear sector have plummeted from to in the past decade (Fig. 3). Landings by gillnets and line gears accounted for over 80% of these landings. Landings by the mobile sector have also been reduced, through reduced TACs, from t in 1984 to in 1992 (Fréchet and Gagnon, 1993). The fishery is conducted year-round. In the winter, otter trawlers exploit the high concentrations of cod that are found in areas 3Pn and southern 4R. Catch rates are usually higher at that time of the year. The fishery is sometimes hampered by ice conditions. In recent years, fish concentrations have been found in much deeper waters than previously (Fréchet and Gagnon, 1991); in 1992, over 90% of the winter survey biomass was found in waters deeper than 360 m compared to less than 5% in the mid-1980s. The summer fishery is prosecuted by a variety of gears with generally smaller vessels than in the winter fishery. In the northern reaches of the Gulf, the fishery often does not begin until mid to late June as ice leaves the area. Generally, over 70% of the landings are made between May and December. Fixed gears account for the majority of the catches from June to September. Rose and Leggett (1988) showed that catch rates of traps are particularly affected by winds that produce periodic upwelling. Southern Gulf of St Lawrence Between 1920 and 1945, landings from the southern Gulf stock varied between and , averaging t per year (Fig. 2). With the general expansion

6 126 G. A. Chouinard and A. Fréchet ICES mar. Sei. Symp., 198 (1994) in the fishery which occurred after 1945, landings increased and peaked at t in The average yearly landings for 1946 to 1970 were twice those of the previous 25 years. Between 1960 and 1975 landings fluctuated between about and t, before declining to their historic low level of t in Landings then increased to about t by the mid- 1980s. Since 1990, landings have declined and reached in 1992 (Sinclair, 1993). Landings have averaged over t between 1971 and Until the mid-1940s, the domestic fishery was almost entirely prosecuted with hook and line gears. Trawlers were introduced in the Gulf fishery in 1947, and in the early 1950s otter trawls became predominant. Fishing with gillnets started in the early 1960s (Halliday and Pinhorn, 1982). A t about the same time, Danish and Scottish seines were introduced. Landings according to gear types are available since Since the early 1980s, catches using fixed gears (gillnets, long-lines, and hand-lines) have declined, attaining their lowest level ever in 1992 (Fig. 3). Many factors likely contribute to this decline, including smaller size-at-age of cod, a shift of some fishermen to the more efficient mobile gears and, in more recent years, the decline in the abundance of cod. The winter fishery, which runs from January to April in 4Vn, is conducted primarily by larger trawlers (>30 m). In recent years, approximately 15% of the catch has been taken during the winter fishery. The fishery in NA FO 4T usually begins when most of the ice has left and cod are returning to the waters of the southern Gulf. This fishery is conducted by smaller vessels (10 to 20 m in overall length). Concentrated fisheries with mobile gears occur in the spring and the fall in the eastern portion of the southern Gulf during the cod migration. Cod are found predominantly in the western portion of the southern Gulf during the months of June to September. Catchability is typically higher in the winter, spring, and fall. The fishery by fixed gears occurs primarily between June and October. This stock has been managed by quota since The TAC for 1993 was initially set at but the directed fishery was subsequently closed on 1 September Trends in abundance, fishing mortality, recruitment, growth, and stock production For these cod stocks, yearly assessment of the abundance of the populations has been conducted for several years, the most recent in May 1993 (Fréchet and Gagnon, 1993; Sinclair, 1993). Fishing mortality and abundance estimates are obtained from sequential population analysis (SPA) calibrated with annual groundfish surveys and, in the case of the southern Gulf of St Lawrence cod stock, otter trawl catch-rate-at-age. N atural mortality is set at a constant 0.2 over all age groups. Northern Gulf of St Lawrence Estimates of removals at age for this fishery are available only for the period since The assessment of the stock uses estimates of cod abundance at age from research vessel surveys conducted in January since 1978 to calibrate the SPA. The age aggregated index from the research survey is shown in Figure 4. Cod are recruited to the fishery at age 3. The results of the assessment indicate that population abundance increased steadily from the mid-1970s to the early 1980s (Fig. 5). It then declined up to the late 1980s and appears to have stabilized in recent years. Total (age 3 + ) biomass has fallen from about t in 1981 to t in 1992, a reduction of 61% (Fig. 6). Adult (estimated as age 7 +) biomass is estimated at in 1992, the lowest level in the period (Fig. 7). Fishing mortality (ages 7 + ) in 1992 is estimated to be 0.51 and has ranged between 0.4 and 0.6 over the time period (Fig. 8). Fishing mortality appears to have been on the increase in recent years. In terms of recruitment (age 3), large year classes were produced in 1974 and 1975, and again in 1979 and 1987 (Fig. 9). While the decline in population abundance is about 50%, the decline in biomass is over 60%. This is in part due to the lower growth rates observed during the 1980s. As an example of the changes seen, the average weight of a cod of age 7 has declined by about 40% since the mid- 1970s (Fig. 10). Reasons for this decline are not well understood. Two year classes comprised 55% of the removals in These are the 1986 and 1987 year classes at ages 6 and 5 respectively in They are preceded and followed by below-average year classes. Given that the estimates of recruitment and biomass in recent years are low, it is likely that recovery of the spawning-stock biomass to average levels observed previously could take several years. Southern Gulf of St Lawrence SPA was calibrated using a research vessel index of abundance derived from the September groundfish survey of the southern Gulf of St Lawrence and catch rates at age from the otter trawl fishery. For this stock, retrospective patterns (i.e. the tendency of current assessments to consistently overestimate stock abundance) have been apparent in previous stock analyses (Sinclair et al., 1991). However, methods used in the assessment of the stock in 1993 were successful in eliminating this tendency (see Sinclair 1993 for further details) and these are considered more reliable.

7 ICES mar. Sci. Symp., 198(1994) Fluctuations in the cod stocks o f the G ulf o f St Lawrence 127 Northern Gulf Southern Gulf 0 "I ! < !---1! Figure 4. Normalized abundance indices derived from research vessel surveys in the Gulf of St Lawrence. Surveys are conducted in January in the northern Gulf and in September in the southern Gulf. The research vessel survey abundance index (Chouinard eta l., 1992) (Fig. 4) shows that abundance was low in the mid-1970s, increased to the mid-1980s, and has been declining since Population abundance (Fig. 5) was high in the early 1950s. Given that average catches prior to 1950 were considerably lower, it is likely that the stock had been previously fished at a lower rate of exploitation, which would have resulted in a build-up of the stock. As the fishery expanded, abundance subsequently declined to reach its lowest level in Northern Gulf Southern Gulf g 400 c Q Figure 5. Trends in population abundance (millions) estimated using sequential population analysis (SPA) for the cod stocks of the Gulf of St Lawrence.

8 128 G. A. Chouinard and A. Fréchet ICES mar. Sei. Symp., 198 (1994) 500 o 450 g 400 <n 350 V) « CQ c 200 Northern Gulf Southern Gulf ê 150 ra Q. o Figure 6. Trends in population biomass for the cod stocks of the Gulf of St Lawrence. Population abundance increased dramatically to the mid-1980s and declined rapidly thereafter. Currently, population abundance is estimated to be at the low level observed in the mid-1970s. The same trends can be seen for total biomass and spawning-stock biomass estimated by ages 5 and over (Figs 6 and 7). Total biomass is estimated to have been t in Total biomass declined to t in 1974 then increased up to the mid-1980s before declining thereafter. Total biomass is currently estimated to be about t. Spawning biomass is also at the lowest level observed. Fishing mortality (ages 7 + ) was high ~ «o to 350 (/> Northern Gulf Southern Gulf o o> Q Figure 7. Trends in spawning-stock biomass (SSB) for the cod stocks of the Gulf of St Lawrence. SSB for the northern Gulf stock is estimated using the biomass of age 7 and older; the estimate for the southern Gulf stock is based on ages 5 and older.

9 ICES mar. Sei. Symp., 198 (1994) Fluctuations in the cod stocks o f the G ulf o f St Lawrence Northern Gulf Southern Gulf o 0.80 c Figure 8. Fishing mortality (ages 7+) for the cod stocks of the Gulf of St Lawrence. (>0.8) in and in recent years (Fig. 8). For the remainder of the period, it varied between 0.4 and 0.8. Recruitment in the southern Gulf cod, estimated by age 3 abundance, has varied by a factor of five over the time period (Fig. 9). Large year classes were produced in , , and , and recruited to the fishery three years after. The abundance of year classes produced since 1980 has been continuously declining. The more recent year classes to have recruited to the fishery ( ) appear to be particularly weak. Of all the cod stocks in the Canadian zone, the change in size-at-age has been most pronounced for the 250 «T 200 Northern Gulf Southern Gulf E 150 E I i i Figure 9. Estimates of recruitment (age 3) for the cod stocks of the Gulf of St Lawrence.

10 130 G. A. Chouinard and A. Fréchet ICES mar. Sei. Symp., 198 (1994) 2.5 kg 1.5 Northern Gulf 0.5 Southern Gulf H h H \ i h H h Figure 10. Average weight of a 7-year-old cod from the stocks of the Gulf of St Lawrcnce. southern Gulf cod. The average weight of an age 7 cod has been 50% lower in recent years compared to the mid-1970s (Fig. 10). Similar to the northern cod stock, it is likely that recovery of the spawning-stock biomass could take a few years. Stock production The production of a stock can be partitioned into two parts; production due to recruitment and production due to growth during the year. The two components of production were calculated for the stocks using the method described by Rivard (1982), in which modified cohort analysis equations are used. The results (Fig. 11) show that production of the southern stock was high in the early 1950s and in the late 1970s and early 1980s. Production for the northern Gulf stock was also high in the late 1970s and early 1980s. Trends in the environment It is widely recognized that environmental conditions have an influence on the production and distribution of fish stocks. Cushing (1982) presents a review of many of the mechanisms involved. In this analysis, several indices of climatic conditions that may be relevant to cod stock fluctuations in the Gulf of St Lawrcnce were examined. The list presented is far from exhaustive. For example, time series of salinity, ice coverage, and other environmental variables were not readily available and were not examined. North Atlantic Oscillation (NAO ) Local environmental conditions may ultimately be linked to global atmospheric circulation. In the North Atlantic, an important feature of atmospheric circulation is the North Atlantic Oscillation. Variation in this circulation pattern has been quantified as the difference in atmospheric pressure between a high-pressure system located in the Azores-Berm uda and a low-pressure system in the Iceland-Greenland area. The index is calculated by computing the mean difference in atmospheric pressure between Akureyri, Iceland, and Ponta Delgada, Azores. High N A O index values usually result in an intensification of winter westerly winds in the West Atlantic and hence colder conditions, while low index values are associated with warmer winter conditions. The NAO index shows low values from the early 1950s to 1964 and high values in the early 1970s, mid-1980s, and recently (Fig. 12, top panel). Degree-days (D E G D ) in the Gulf of St Lawrence An index of temperature conditions in the Gulf of St Lawrence was calculated by taking the average of degree-days (Anon., 1992) from six nearshore weather stations (Gaspé, Sept-Iles, Charlo, Summerside, Ilesde-la-Madeleine, and Stephenville) in the Gulf of St Lawrence. These stations have been consistently monitored since at least The resulting index (Fig. 12, second panel) shows that conditions were generally warmer in the late-1960s and in the early 1980s. Cold

11 ic e s mar. sd. Symp., 198 (1994) Fluctuations in the cod stocks o f the G ulf o f St Lawrence (a) Northern Gulf B O g o a Recruitment Growth t i l l S (b) Southern Gulf Recruitment Growth a 3 a o QL Figure 11. Growth and recruitment components of production for (a) the northern and (b) southern cod stocks of the Gulf of St Lawrence. conditions were prevalent in the early 1970s and in some recent years. Bottom temperatures in the Gulf of St Lawrence Bottom temperatures from groundfish surveys that have been conducted in the Gulf of St Lawrence were also examined. In the northern Gulf, a survey conducted in January of each year since 1978 (with the exception of 1982) indicates that bottom temperatures were cooler in the early 1980s and in recent years (Fig. 12, third panel). In the southern Gulf, the survey has been conducted each September since Generally, bottom temperatures in the area at that time of the year were warm in the late 1970s. Cabot Strait deep-water temperature The changes in deep-water temperatures in Cabot Strait have been examined by Bugden (1991). The waters are thought to be a mixture of the Labrador Current and slope waters in essentially equal proportions. These waters have been shown to propagate upstream following a typical estuarine circulation pattern and reach the

12 1 3 2 G. A. Chouinard and A. Fréchet ice S m ar. sd. symp., 1 9 8(1994) NAO Index ( Degree-Days cn. A.\ \ > '\ V Degrees C ) A *» **'* v \ ; V \ : \ ;\ \ : \ ; \ V Southern Gulf Cabot Strait +- - Northern Gulf RIVSUM o Figure 12. Plot of some environmental variables for the Gulf of St Lawrence. Top panel: Index of the North Atlantic Oscillation (dotted line) and running mean (solid line). Second panel: Mean degree-days of air temperature for six stations in the Gulf of St Lawrence (dotted line) and running mean (solid line). Third panel: Average midwater and bottom temperatures from research surveys conducted in the Gulf of St Lawrence (see text for details). Bottom panel: Index of freshwater discharge (hundreds of m3/s) from the St Lawrence River (dotted line) and running mean (solid line).

13 ICES mar. Sei. Symp., 198 (1994) Fluctuations in the cod stocks o f the G u lf o f St Lawrence 133 St Lawrence estuary in approximately 2.5 years. The annual means of water temperatures at m were calculated. Generally, the index shows that temperatures were low in the mid-1960s, before increasing to the early 1980s and then decreasing again (Fig. 12, third panel) St Lawrence River Discharge (RIVSUM ) Sutcliffe (1973) found significant correlations between some fisheries resources landings and the freshwater discharge from the Gulf of St Lawrence. He hypothesized that freshwater discharge promoted nutrient enhancement of the top layer waters through mixing and entrainment processes. The index of river discharge used was the sum of monthly mean discharge from several gauging stations on the St Lawrence River (Lake Ontario, the Ottawa River, and the Saguenay River) and is referred to as RIVSUM. In the 1960s the index accounted for about 70% of the total discharge from the St Lawrence River in the estuary. The RIVSUM index shows a progressive decline in river runoff from 1950 to the mid-1960s (Fig. 12, bottom panel). Discharge subsequently increased, attaining its highest level in the mid-1970s before declining slightly and fluctuating in the early 1980s. In 1989, the index was at the low levels of the early 1970s. Causes of fluctuations: fishing or recruitment? From a given level, the abundance of a fish population will fluctuate if the removals (fishing and natural mortality) do not equal the incoming recruitment. Mann (1993) concluded that, in many cases, fluctuations in the abundance of fish populations are due to large-scale oceanographic processes which affect year-class size rather than exploitation regimes. This hypothesis was provisionally examined for the two cod stocks of the Gulf of St Lawrence. The influence of the variation in the exploitation rate (fishing mortality) and recruitment on population trends for the two stocks were investigated by conducting population simulations (Rivard, 1982) under two conditions. These were in effect stock predictions starting in the 1970s using different values of recruitment and fishing mortality. The population trends obtained from these projections were then compared with the estimated stock trends derived from the 1993 assessments of these stocks. A similar approach was used by Fréchet ( 1991 ) to examine the dynamics of the northern Gulf cod stock under various scenarios of exploitation. Given an estimated initial population at age A and in year Y (NA.Y)> the population at age A + 1 and year Y + 1 were estimated using: n a + i.y + i = n a,y e x p (FA.v+ M A.Y> where: FA Y = fishing mortality at age A in year Y and M a,y = natural mortality at age A in year Y. The numbers in the first age group were set equal to the recruits. The periods examined were for the southern Gulf of St Lawrence stock and for the northern Gulf stock. From the results of the most recent assessment of these resources (Fréchet and Gagnon, 1993; Sinclair, 1993), the mean recruitment and the mean fishing mortalities over the time period for each age class and for each stock were calculated. These mean values were used in simulations using constant conditions of either recruitment or fishing mortality. The estimated values of initial population, fishing mortality, and recruitment were those derived from SPA from the most recent stock assessments of these stocks (Fréchet and Gagnon, 1993; Sinclair, 1993). Starting with the initial population estimates at the beginning of these time periods (1971 for the southern Gulf and 1974 for the northern Gulf), population predictions were then conducted under two scenarios. In the first scenario, the estimated recruitment values from SPA and the mean fishing mortality at each age were used in the prediction. In the second scenario, the mean recruitment and the estimated fishing mortality from SPA were used. Natural mortality was set at 0.2. The results (Fig. 13) were compared with the current estimates of population abundance for these stocks (i.e. those resulting from estimated fishing mortalities and recruitment values). They indicate that population abundance would have followed the same trends even if fishing mortality had remained constant over time, given the recruitment estimated. However, if we assume constant recruitment and use the estimated fishing mortality from SPA in the prediction, trends in population abundance do not match those estimated. This indicates that the year-to-year fluctuations in population abundance depend more on the variation in recruitment than on the variation in fishing pressure for these stocks. This is not to say that fishing had no impact on these resources, but rather that the fluctuations (as opposed to the actual level) in population abundance seen over the last 20 years or so have not been caused primarily by the variation in mortality at the exploited stage. However, in the case of the southern Gulf of St Lawrence (see bottom panel, Fig. 13), the recent increase in fishing mortality seems to have exacerbated the rate of decrease in population abundance. The coefficient of variations of the estimated recruitment and total mortality values from SPA also provide an indication of which factor has more influence on the

14 134 G. A. Chouinard and A. Fréchet ICES mar. Sei. Symp., 198 (1994) <A i» 0) n E ~ I! o = += c ro S- 3 D. O CL (a) Northern Gulf ^ Estimated F and Constant F and Estimated F and Constant R Estimated R Estimated R (A v. a> IS II O = ro, 3 Q. O CL (b) Southern Gulf Estimated F and - Constant F and Estimated F and Constant R Estimated R Estimated R Figure 13. Results of simulations conducted with constant (i.e. average) and estimated (i.e. from SPA) values of recruitment (R) and fishing mortality (F) for (a) the northern and (b) southern cod stocks of the Gulf of St Lawrence. Starting population values were the population size estimated from SPA at the beginning of the time period and M was set at 0.2 (see text for details). population trends. Recruitment tends to be more variable than the level of removals by the fishery (Table 1). We conclude that in the case of these two stocks the fluctuations in population abundance have been largely caused by fluctuations in the recruitment. These fluctuations can be intensified by variation in fishing mortality. Survival of pre-recruits and climatic conditions As in the case of most other cod stocks, the stock-recruit relationships for the two cod stocks of the Gulf of St Lawrence do not conform with theoretical models of stock and recruitment (Fig. 14). It could, in fact, be argued that there is a negative relationship for the northern Gulf (Fréchet, 1991) or that we may have witnessed only the descending limb of a stockrecruitment relationship. As several authors have pointed out (Sætersdal and Loeng, 1987; Koslow et al., 1987; W yatt et al., 1992), there are a number of environmental variables affecting the relationship. In this context, the notion of survival might reconcile the apparent lack of relationship between spawning stock and recruitment. lies (1973) suggested an index of survival of pre-

15 ICES mar. Sei. Symp., 198 (1994) Fluctuations in the cod stocks o f the G u lf o f St Lawrence 135 Table 1. The coefficients of variation of the estimated recruitment and total mortality values from SPA. Coefficient of variation (%) Recruitment Total mortality (ages 7+) Northern Gulf (74-92) Southern Gulf (71-92) recruits by dividing the estimated recruitment (in this case, obtained from SPA) by the spawning stock biomass (SSB) which produced it. The SSB is seen as an index of the number of eggs produced, the recruitment in later years providing an index of how many survived. Pre-recruit survival for both stocks indicates that there was exceptional survival in the mid-1970s (Fig. 15). Survival during that period was about an order of magnitude higher than the lowest levels observed for the 200 «T c 0 = c 100 a> + * E Ë 50 o a> DC 0 77 (a) Northern Gulf ' '» *76 * » *83 *89 *85.84 C) Spawning Stock Biomass ( 0001) Recruitment (millions) m ro ai en cn o o o o o o (b) Southern Gulf ,77 "73,64 82» ^65,83, *51.52 *^ 2V ' 62 *58 ) Spawning Stock Biomass ('0001) Figure 14. Plots of spawning-stock biomass and recruitment for (a) the northern and (b) southern cod stocks of the Gulf of St Lawrence.

16 136 G. A. Chouinard and A. Fréchet ICES mar. Sei. Symp (1994) 2.5 re! 1.5 </) o X 1 <D TJ C 0.5 Northern Gulf Southern Gulf Figure 15. Index of survival (Reeruitment/SSB) for the cod stocks of the southern Gulf of St Lawrence. stocks. The year classes in recent years produced by large biomasses have had lower survival. Pearson correlation coefficients between this index of survival and the environmental variables RIVSUM, N AO, and degree-days (during the birth year) were calculated for the southern Gulf stock. O ther environmental variables were not examined, as it is hypothesized that survival would be influenced by conditions in the near-surface during the first year of life. For the northern Gulf stock, RIVSUM was not used as general X fl> TO 1.50 ro 'j> </) #82 " 71» 88 * Il RIVSUM _ *86 Figure 16. Plot of the index of discharge from the St Lawrcnce River (RIVSUM) and the index of survival for the southern Gulf of St Lawrence cod stock

17 ICES mar. Sei. Symp., 198 (1994) Fluctuations in the cod stocks o f the G u lf o f St Lawrence 137 circulation patterns indicate that the outflow from the St Lawrence River is predominantly on the southern Gulf. Results (see Table 2) indicated that the index of survival is weakly correlated with freshwater discharge but not with degree-days for the southern Gulf stock. A plot of the data (Fig. 16) shows that there are probably other factors at play as there have been some low survival rates associated with above average discharges; however, very high survival was only achieved when freshwater runoff was above average. Discussion and conclusion Trends in landings in the cod fisheries of the Gulf of St Lawrence were relatively similar. Generally, they show the same trends seen in the adult population biomass. Since 1974, the management objective has been to exploit the stocks at a constant target fishing mortality ( F m a x then F01) (Parsons, 1993). Despite the fact that the targets were not achieved, estimated fishing mortality was maintained at a relatively stable level and it is not surprising to see trends in landings to be synchronous with exploitable biomass. However, for the southern Gulf stock, the trends in landings prior to 1974 also follow trends in biomass quite closely, despite the fact that the fishery was less regulated. Fishing mortality gradually increased between 1950 and the early 1970s but growth also increased between the early 1960s and early 1970s, with the result that trends in landings followed the trends in biomass. The two cod stocks exhibit relative synchrony in their stock status trends. Generally, the mid-1970s and recent years were periods of low abundance for both stocks, while abundance was high in the mid-1980s as a result of the large year classes that were produced in the late- 1970s and early 1980s. Our results show that, for these two stocks, population abundance fluctuations were largely determined by short-term variations in the recruitment as opposed to those in fishing mortality. Almost certainly, if fishing Table 2. Results of correlation analyses of the index of survival for the northern and southern Gulf of St Lawrence cod stocks and selected environmental variables. r P n Northern Gulf NAO DEGD Southern Gulf NAO DEGD RIVSUM mortality had been lower on average, population abundance would have been higher. Nevertheless, it would likely have fluctuated in response to change in recruitment levels, albeit at a higher level. O ur analysis did not include feedback from an undefined underlying stock-recruitment relationship. However, given that fishing mortality did not fluctuate much over the time period, the impact on the results would probably be minimal. Koslow et al. (1987) showed that there were significant correlations between recruitment of several cod stocks in the Northwest Atlantic. Cohen et al. (1991) indicated that the correlations were usually strongest between neighbouring stocks. Given that similar trends in abundance are also seen for most other cod stocks in the Canadian Atlantic, with the exception of Georges Bank and Bay of Fundy stocks (Sinclair, 1993), it would appear that fluctuations in abundance may be related to fluctuations in recruitment as opposed to fluctuations in fishing mortality for most of these as well. The impact of physical forcing has long been thought to have a major impact on the recruitment of fish stocks (Cushing, 1982). The indices of temperature examined indicate some similarities. For example, the values of the N AO index roughly correspond with periods of lower and higher degree-days in the Gulf of St Lawrence. As expected, bottom temperatures, which are influenced by factors such as the degree of stratification or inflow of cold waters from the Atlantic, do not match the trends well. The problems with correlations of environmental variables and fish stock recruitment have been highlighted by several authors (Walters and Collie, 1988; Cohen et al., 1991). Statistical theory states that if numerous correlations are done, some are bound to be significant. As Mann (1993) points out, more often than not the relationships eventually cease to hold, probably because a series of other factors with different levels of importance intervened. Correlations can nevertheless be useful to establish hypotheses that can be tested with the proper experimental design. The correlations of the index of survival and the environment examined here do not include all the possible effects, such as other environmental variables (salinity, etc.) and predator-prey relationships. For example, Lett (1980) hypothesized interactions of cod and mackerel in the southern Gulf. In our analyses, the only significant correlation found was for the index of survival for the southern Gulf stock and the freshwater discharge of the Gulf of St Lawrence. Therriault and Levasseur (1986) have shown that the primary productivity of the St Lawrence estuary was influenced by freshwater discharge. Thördardöttir (1986) described a similar effect in the Icelandic coastal current. Skreslet (1976) found that the survival of juven-

18 138 G. A. Chouinard and A. Fréchet ICES mar. Sei. Symp., 198 (1994) ile Areto-Norwegian cod hatched on the north coast of Norway was positively correlated with freshwater discharge on the southwest coast of Norway the previous year. Bugden et al. (1982) previously examined the effect of the Gulf of St Lawrence freshwater runoff on year-class variability of cod in the Gulf of St Lawrence, concluding that the effect was weak. The relationship found here is not strong either; in some years, high discharge did not lead to higher survival; however, high survival was only observed when freshwater discharge was above normal. This probably implies that other factors are at play in the mechanism(s) involved or that the freshwater discharge is simply a proxy for some other condition, such as the degree of mixing of nutrients or even primary and/or secondary production. Several predictions of global warming have been prepared for Canada (Kemp, 1991). One prediction, which relates to freshwater discharge, is that the flow from the G reat Lakes basin to the St Lawrence would be reduced by about 20% (Sanderson, 1987). The relationship between the index of survival for cod in the southern Gulf of St Lawrence and the discharge from the St Lawrence is admittedly weak. However, this may mean that under the same other conditions that prevailed in the mid- 1970s, higher survival than average could be less likely. Although the recruitment component of production appears to be affected, the growth component may not be affected in a similar way or by the same variable(s). For the southern Gulf of St Lawrence, Hanson and Chouinard (1992) have shown that changes in size-atage are consistent with size-selective fishing mortality. Density effects have also been suggested with smaller size-at-age observed at high population abundance (Paloheimo and Kohler, 1968; Chouinard and Hanson, unpublished data). For example, the year classes produced in the late 1960s and early 1970s, which are estimated to be small, attained an average weight of about 2.5 kg at age 7, the largest observed in the time series. Conversely, cod produced in 1980, the largest year class on record, barely reached a weight of 1 kg in Density has also been associated with changes in distribution (Swain and Wade, 1993). A t high densities, the geographic range of southern Gulf of St Lawrence cod increased. Changes in the bathymetric distribution patterns related to density have also been observed (Swain, 1993). At high densities, cod were found in deeper waters. There is evidence that climatic fluctuations can also affect migration and distribution of cod stocks. For the Gulf of St Lawrence stocks, Fréchet (1990) showed that concentrations of overwintering cod from the northern Gulf stock are found at the marginal ice zone. The results of the study showed that cod were virtually absent within the ice field. The increase in ice coverage seen since 1990 was therefore likely responsible for the extended migration of the southern Gulf stock into area 4Vs in recent years (Hanson et al., 1991). It is also possible that a similar extension of the migration of the northern Gulf stock happened as well. Up until recently, Canadian stock assessments have focused their efforts on estimating fishing mortality and abundance as opposed to trying to predict the size of incoming year classes. Our analysis shows that efforts to better estimate both current and incoming recruitment may be more helpful in predicting stock trends, providing that some control on fishing effort remains in place. As for the mechanisms that determine the abundance of the year classes, these are no doubt complex and, in addition, the relative importance of the various factors may be changing with changes in the environment. Acknowledgments K. Drinkwater provided the Cabot Strait deepwater information and the NAO index. 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