FISHERY HARVEST AND POPULATION DYNAMICS OF GRAY SNAPPER, LUTJANUS GRISE US, IN FLORIDA BAY AND ADJACENT WATERS

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1 BULLETIN OF MARINE SCIENCE, 44(1): , 1989 FISHERY HARVEST AND POPULATION DYNAMICS OF GRAY SNAPPER, LUTJANUS GRISE US, IN FLORIDA BAY AND ADJACENT WATERS Edward S. Rutherford, James T. Tilmant, Edith B. Thue and Thomas W Schmidt ABSTRACT Catches of gray snapper, an important recreational gamefish species in south Florida, have been monitored nearly continuously since 1958 in Everglades National Park; total harvest and effort data have been collected since 1973, and lengths have been measured since Catch rates of gray snapper have fluctuated greatly since 1958 with peaks in 1959, , and Most of the total annual harvest from 1973 to 1985 was taken by sport fishermen (78%) and guided parties (21%) with the remaining I% taken by commercial hook-and-line fishermen and net fishermen. Total annual harvest of gray snapper in Florida Bay and adjacent waters dropped from 129,000 to 99,500 fish between , increased greatly to 156,000 fish in the mid-1970's, but declined again during the 1980's to 59,000 fish. Effort was linearly correlated with harvest (r' = 0.973, N = 13). The great increase in harvest in the mid-1970's was due to a great increase in guide harvest. The decline in effort, harvest, and harvest rates for gray snapper since 1979 is believed due to increased effort for other species such as spotted seatrout, as well as reduced stock abundance and recruitment. Gray snapper recruit to the park fishery at age I and are found in the catch to at least 7 years. Three and 4-year-old fish make up 87% of the catch. Gray snapper are believed to migrate offshore out of the park to spawn since very few ripe adult fish have ever been found in the park. Gray snapper along the keys and Florida's east coast live to at least 21 years old. Although fishing mortality on gray snapper in the park is high, averaging F = 0.76, and the stock is growth-overfished, population size and recruitment are not controlled by fishing effort within the park. Environmental factors and possibly fishing effort on gray snapper in the adjacent Florida Keys may control stock size. Fishing activity and harvest of sportfish from Everglades National Park (ENP) have been monitored nearly continuously since Concern over declining catches and catch rates of gray snapper (Lutjanus griseus) and other popular gamefish species in ENP in the 1970's prompted the National Park Service (NPS) to enact bag limit restrictions in 1980 that limited catch ofany species to 10 fish' person day-i and no more than 20 fish (species combined' person' day-i). Commercial fishing of any type was eliminated from the park as of Controversy over the bag limit regulations prompted the Assistant Secretary for Fish and Wildlife and Parks, U.S. Department of the Interior, to direct the U.S. Fish and Wildlife Service and the NPS to evaluate the effect of fishing on the harvested marine resources of the park. Previous studies (Higman, 1967; Davis, 1980) indicated that environmental factors affected sport fish catch rates more than fishing. We present in this report the population status of gray snapper in the Florida Bay portion of ENP and adjacent waters and its response to fishing effort and rainfall. The gray snapper is a lutjanid species that has always been one of the four most popular gamefish species in ENP and comprises the highest harvest in the recreational fishery. Gray snapper enter the park as small juveniles, grow up in park waters to age 3-4, and then start leaving the estuary to spawn and live on offshore reefs (Rutherford et al., 1983). The oldest fish found in ENP was 7 years old but 139

2 140 BULLETIN OF MARINE SCIENCE, VOL. 44, NO. I, 1989 t.4 t n ll [] HARVEST RAl<: YE'AR + CATCH RAl<: Figure I. Sportfishermcn catch and harvest/man-hour of gray snapper in the Florida Bay area, offshore they live to at least 21 years (Manooch and Matheson, 1983; Rutherford et ai., 1983). Description of the Study Area. - The mainland shoreline of Everglades National Park extends from the Florida Keys to Everglades City on Florida's west coast. Tabb et al. (1962) have described the animal and plant communities of park waters and identified distinct ecological zones. Their work provided the basis for delineating fishing areas used in this study and other Everglades National Park fishery investigations since 1960 (Higman, 1967; Tilmant, 1989, fig. 1). These areas vary in their topographical, hydrological, and biotic characteristics. METHODS Methods for the park recreational and commercial fishermen surveys conducted by the NPS and the University of Miami are described by Tilmant (1989). These surveys provided estimates of catch and harvest (fish landed) per-unit-effort for successful sport fishermen (those fishermen who caught gray snapper) since 1959, catch and harvest rates for successful commercial fishermen since 1972, total estimated harvest and effort for all fisheries since 1973, and length measurements since Statistically significant differences in mean catch rates of gray snapper over time or betwcen fishing areas were determined by analysis of variance of 10glOtransformed rates. Effort Standardization. - Harvest rates were standardized among gear types, seasons, and areas by species using Robson's (1966) method of regression analysis. Total estimated effort for a species was then standardized by dividing the total harvest of all gcar types by the standardized harvest rate. Tota/ Mortality Estimates. - Total instantaneous mortality rates were estimated for gray snapper from catch curves obtained from creel-survey length measurements made from 1974 to Fish lengths were split into age classes using an age-length rclationship developed in a previous park age and grow1h study (Rutherford et ai., 1983). Total instantaneous mortality rate (Z) was estimated for each year by regression that log.,transformed frequency of fully recruited fish against age. The absolute value of the rcgression slope is Z. Age at full recruitment in this analysis was taken to be the first age after the dominant year class in the sample.

3 RUTHERFORD ET AL.: GRAY SNAPPER 141 '".. 3D D 10 v.«i.,i2u ~ 727J I II aui :514 r ,,717111:I' Figure 2. The percent of sportfishing boats interviewed who caught gray snapper (left) and spotted seatrout (right) in the Florida Bay area, Virtual Population Assessment (VPA).-A virtual population assessment (VPA) was used to provide estimates of abundance, recruitment, fishing mortality rate, and catchability coefficient for gray snapper. The program COHORT, written by Fox (1976) and modified for personal computers by David Die and Victor Restrepo of the University of Miami Rosenstiel School of Marine and Atmospheric Science, was used to run the VPA. COHORT solves the catch equations of Murphy (1965) iteratively, and requires estimates of harvest for each age, natural mortality rate (M), total fishing effort, and estimates of thc fishing mortality rate of the oldest age in a cohort. Catch (harvest) by age class was detennined for gray snapper by applying the proportional distribution of age frequencies determined from length measurements observed in the fishermen catches to total estimated harvests from 1974 to An estimated natural mortality rate of M = 0.30 for gray snapper was obtained from an age and growth study of the Florida east coast and the Florida Keys (Manooch and Matheson, 1983) using Pauly's (1980) equation relating M to von Bertalanffy growth parameters K, L, and ambient temperature. The rate of 0.3 was applied to gray snapper ages 1-4 and increased for ages 5-7 to account for an unknown but postulated migration by adult fish out of the park to offshore reefs. The amount of standardized total fishing effort to which the stock was subjected each year was assumed to apply equally to fully recruited fish. An iterative fitting procedure was used to estimate the terminal F (fishing mortality rate during oldest available age of a year class) for the VPA. The terminal F for cohorts in 1985 was chosen so that the F values generated by the VPA for fully recruited ages provided the best match with the effective effort estimated above. It was assumed that the terminal F for each cohort (i) which was fully recruited in 1985 had a constant fishing mortality rate (F T ). The catch equations below were then applied to determine N;,+I, a, ll2. 8J M Figure 3 (left). Harvest per successful sportfishing boat of gray snapper in the Florida Bay area, Figure 4 (right). Recreational and guide fishermen harvest rates of gray snapper,

4 142 BULLETIN OF MARINE SCIENCE, VOL. 44, NO.1, too ~I: gl.. '" 20 10, , ". ". t" ". ~ t211 H :1 :.. '" n ~ ~ n n n ~ ~ 81 ~ ~ IS:] ~l1otw. Ia::JGUIDE """ ~ HOOK LINE ls2l NET 60 8Q ,.. :roo sr~)(w,i,n-hits) Figure 5 (left). Total estimated harvest by fishery of gray snapper in the Florida Bay area, Figure 6 (right). Estimated total harvest vs. standardized effort for gray snapper in the Florida Bay area, the number of fish at the beginning of year t + I, N,,, the number of fish at the beginning of year t, and F" the fishing mortality rate at the beginning of year t. Nit = Ci,(FT + M)/FT(1 - exp(-ft-m)] Cit = Nit+,(exp(FT + M] - I)/(FT + M) N"+I = Ni,exp(Ft + M) ( I) (2) (3) The F values for the terminal ages in were obtained by averaging the VPA-generated fishing mortality rates of age at recruitment through the terminal age - I for each year starting in 1984 and sequentially working backward in time to The iterative fitting procedure included years and was limited to the cohorts in 1985 for which the terminal F was estimated. A range off.'s was searched to minimize the criteria: 1984 term mal age RES2 = ~ ~ (f;, - f.f t~ 1974 J=recruit age (4) Where f. is the observed effort on recruited fish in year t and f;, is estimated effort on age "i." Estimated f;, was found by regressing: where F't are age-specific F values from the VPA. Thus: (5) f;t = Fi/qt (6) The terminal F's of pre recruited fish in 1985 were estimated assuming the 1985 F values were equal to the age-specific averages in earlier years. This assumption means that the recruitment estimate for 1985 is imprecise. Table 1. Estimated total harvest by fishery of gray snapper in the Florida Bay area, Fishery Recreational 123, , ,008 95, ,081 94,833 89,433 Guides 5,672 4,897 3,036 3,412 34,375 31,985 60,308 Hook-and-line , Net ,965 1,170 3,202 Total 129, , ,360 99, , ,347

5 RUTHERFORD ET AL.: ORA Y SNAPPER 143 Catchability Coefficient q- The catchability coefficient q was obtained as the slope from regression equation 5. Yield-Per-Recruit Analysis. - The Beverton and Holt yie1d-per-recruit equation (Ricker, 1975, 251) was used to assess impact of fishing pressure and size limits on yield (pounds of fish harvested) for gray snapper. The equation assumes constant recruitment and isometric fish growth. It is unknown whether gray snapper within areas 1-5 represent a different stock than gray snapper from the upper Gulf Coast. All tagging studies on gray snapper in south Aorida indicate this species is relatively non-migratory, moving less than 20 miles from point of release (Beaumariage, 1969; Nicholas Funicelli, pers. comm.). We have, therefore, assumed the Aorida-Whitewater Bay population comprises one unit stock for our analysis. RESULTS Catch Rates. - Between 269 and 2,319 recreational fishing parties a year (mean = 1,213) reported catches of gray snapper. Gray snapper was most common in the catches during late summer (July-September) and least common during winter (January-March) for all years except 1981 when gray snapper catches decreased markedly. When fisheries monitoring in Everglades National Park was first initiated in 1958, at Flamingo, the principal sportfishing interview site for the Florida Bay area, gray snapper populations were apparently high with over 1.1 snapper harvested' h- I of fishing (Fig. I) and over 40% of the fishing boats catching this species (Fig. 2). Harvest rates rapidly dropped, however, to less than 0.65 fish' man-h- 1 effort by During the late 1960's, the fishery recovered somewhat and annual average harvest rates ranged from 0.85 to 1.0 I fish' man-h- 1 During the 1970's, gray snapper populations appeared to increase within the park as harvest rates increased annually through Since 1979, however, the annual mean harvest rate of successful fishermen has declined steadily (Fig. 1). The percent of boats catching gray snapper showed an interesting inverse relationship with the percent of boats catching spotted seatrout, suggesting that sportfishermen may target other species than gray snapper when fishing is good (Fig. 2). The gray snapper harvest rate of guide fishermen followed the trend in the recreational fishery (Fig. 3). Harvest per boat of gray snapper declined even more markedly than HPUE statistics indicate (Fig. 4). One factor that partially explains the lower harvest per boat is that the average man-hours fished per party successful for gray snapper has decreased from a range of man-h trip-l for the early sixties to an average of only 9.7 man-hours in Therefore, even though the catch and harvest per hour fished was higher in 1984 than 1961, the number offish harvested per trip was not higher. The reduced man-hours per trip and number of fish harvested per trip may largely be due to the 10 fish per species bag limit imposed on the fishery in Table I. Extended Avg 59,277 44,172 41,798 38,882 47,813 47,469 57,648 78,504 33,298 21,304 17,401 19,417 27,559 34,227 19,032 21, ,945 65,865 59,552 58,738 75,714 81,856 76,680 97,394

6 144 BULLETIN OF MARINE SCIENCE, VOL. 44, NO. I, 1989 Table 2. Proportion of harvest, catch, and effort of all species taken by guide fishermen by major species in ENP (H = % total harvest, C = % total catch, and E = % total successful guide effort) Snook Tarpon Gray snapper Spoiled seatrout H C E H C E H C E H C E I I I I R% S SS Harvest/Effort. -A summary of the estimated total annual harvest of gray snapper from the Florida Bay area by all fisheries is shown in Table I. The total annual harvest of gray snapper ranged from less than 58,750 fish in 1983 to over 156,213 fish in 1977 (Fig. 5). Over the period of record, 78% of the total harvest of gray snapper was taken by the recreational fishery, while 21% was taken by guides, and less than I% of the total harvest was taken by commercial hook-and-line or net fishermen (Table I). Annual recreational harvest of gray snapper declined steadily since 1973 (except 1977) even though catch and harvest rates increased during the late 1970's (Table I, Fig. 1). Two factors account for this decline. During the 1970's when gray snapper harvest rates increased, the number of fishermen coming to the park declined drastically and, therefore, even though the harvest rate of successful Table 3. Estimated total standardized effort by fishery for gray snapper in the Florida Bay area, (seasons are W = Jan-Mar; SP = Apr-June; SU = Jul-Sep; F = Oct-Dec) Fishery Seasons Rec. stand. effort W 25,644 15,469 21,759 22,258 29,777 Man-h harv. SP 28,993 29,598 34,274 19,056 24,370 SU 27,215 41,908 34,156 27,884 48,594 F 40,272 28,950 28,817 30,074 32,786 Total 122, , ,006 99, ,528 Guide stand. effort W 1, ,091 1,862 13,790 Man-h harv. SP 971 1, ,144 SU 372 1, ,264 F 1, ,625 Total 4,360 4,749 2,874 30,894 32,823 Total stand. effort W 27,182 16,299 22,850 24,120 43,568 Man-h harv. SP 29,964 31,543 35,196 19,408 33,514 SU 27,587 43,583 34,869 28,254 54,858 F 41,750 29,249 28,965 30,574 36,412 Overall total ]26, , , , ,351

7 RUTHERFORD ET AL.: GRAY SNAPPER 145 Table 2. Extended Black drum Red drum Bonefish Sheep,head H C E H C E H C E H C E I I fishermen increased, less effort led to lower total harvest. Secondly, since fishing activity increased ( ), catch and harvest rates dropped markedly leading to a continued decline in annual gray snapper harvest. Gray snapper accounted for 24% of the cumulative catch, 23% of the cumulative harvest, and 31% of the total effort for all species taken by the guide fishery, but these percentages have decreased markedly since 1980 (Table 2). Gray snapper was the only guide target species that declined in percent of the total guide harvest, catch, and effort (Table 2) since Gray snapper is most probably the species harvested when guide parties are not successful for trophy species such as tarpon or bonefish or prized food-fish such as snook or red drum. This point is illustrated by comparing the average percent total guide fishing effort (Table 2) for versus : gray snapper effort declined 34% to 25% while effort for Table 3. Extended ,379 19,958 23,078 20,010 11,962 7,378 8,071 10,385 27,508 20,611 18,454 12,271 9,491 13,335 12,948 15,184 35,650 34,099 18,139 9,067 17,239 22,562 21,624 15,968 31,237 33,755 16,125 14,289 12,466 12,726 13,442 13, , ,422 75,796 55,637 51,158 56,002 56,085 54,744 4,327 12,407 7,360 9,737 6,618 5,502 5,664 8,912 8,417 18,142 8,391 4,785 4,251 6,588 12,495 8,853 3,653 17,246 9,894 3,657 1,918 3,697 4,986 10,512 10,906 9,579 6,771 2,126 3,738 3,318 4,238 5,722 27,302 57,374 32,416 20,305 16,524 19,105 27,383 33,998 19,706 32,365 30,439 29,747 18,580 12,880 13,736 19,296 35,925 38,753 26,845 17,056 13,742 19,923 25,443 24,037 39,302 51,344 28,033 12,724 19,157 26,259 26,610 26,479 42,143 43,334 22,895 16,416 16,204 16,045 17,680 18, , , ,211 75,942 67,682 75,107 83,468 88,743

8 146,.. "" "" = > JOO '" ~ "" I "" '00 ~...,<0 "" " BULLETIN OF MARINE SCIENCE, VOL. 44, NO.1, L 989 ~.. o FROWCTCH~ Figure 7 (left). Mean length (mm F.L.) of gray snapper in the Florida Bay area, Figure 8 (right). Total instantaneous mortality rates of gray snapper estimated by catch curves and VPA, Diamonds represent upper 95% Confidence Intervals for Z estimated by catch curves. tarpon (6% to 9%), bone fish (2% to 3%), and red drum (36% to 53%) increased. The steady increase in harvest and lack of a noticeable decline in effort since 1976 for spotted seatrout and red drum indicate that they may be a more desired target for guide fishermen. Effort-Harvest Relationship, - The estimated standardized fishing effort of each fishery on gray snapper is shown in Table 3. Annual standardized effort ranged from a high of 168,351 man-hours in 1977 to a low of67,682 man-hours in Eighty-three percent of the total standardized effort was made by the recreational fishery while 16% has been by guides and less than I% by hook-and-line or net fishermen. A highly significant linear relationship (r 2 = 0.973, N = 13, P < 0.001) was found between the estimated total standardized effort placed on the fishery each year and the resulting harvest (Fig. 6), suggesting that the rate of exploitation of gray snapper within the park has been moderate. Size.-A total 11,626 gray snapper have been measured since 1974 from the recreational catch (mean = 1,057 'yr- l ). Recorded sizes have ranged from mm F.L. and averaged 264 mm (SD 50 mm) overall. Mean lengths of gray snapper in 1979, 1981, 1984, and 1985 were significantly (P < 0.05, SNK test) smaller than mean lengths of other years (Fig. 7). Significant differences (SNK test) were found in the mean length of gray snapper among seasons and areas of capture during most years (Table 4). The largest annual mean size of fish were caught more often from Cape Sable and the Shark River areas than from any other areas (Kruskal-Wallis H = 12.75, P < 0.05). The smallest annual mean length for gray snapper occurred significantly more often in Whitewater Bay and northern Florida Bay than other areas. The mean length of gray snapper was largest during either the spring or summer period for every year except 1974, 1975, 1978, and However, these two seasons were significantly larger than either winter or fall only in 1980 and The smallest mean lengths occurred during either fall or winter every year except 1974 and 1975, but, again, differences were not statistically significant from either spring or summer in most years (Table 4). Age of Catch. - Estimated total annual harvest by age is shown in Table 5. Gray snapper begin entering the fishery at age 1 and are fully recruited by age 4. Nearly 87% of the harvest are age 3 and 4-year-old fish.

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10 148 BULLETIN OF MARINE SCIENCE, VOL. 44, NO. I, 1989 Table 5. Estimated total harvest by age of gray snapper in the Florida Bay area, Age I 2, ,026 2,723 3,920 7,779 7,257 23, ,033 48,479 39,529 60,486 58,851 64, ,854 45,013 39,857 64,594 47,861 47, ,986 9,657 7,840 15,825 11,042 9, ,230 2,231 7,154 4,108 4,555 5, ,628 1,170 1,025 2, ,181 Totals 118, ,360 99, , , ,347 Mortality Rates. -Annual total mortality rates based on catch curves are shown in Figure 8. Total instantaneous mortality rates (Z) ranged from 0.87 to 2.03 (mean = 1.29) with large confidence intervals (95%) and no significant annual differences. Highest estimated mortality occurred during 1980 and lowest estimates occurred in During all other years mortality rate estimates varied only from 1.10 to As with harvest rates, annual total mortality rates were not significantly correlated with the amount of harvest (same year or any mean of previous years) or total standardized effort (same year or mean of any previous years), suggesting that recruitment, immigration-emigration, or natural mortality were more dominant than fishing effort in determining age composition of the population. Yield-Per-Recruit. - The response of yield to various levels of fishing mortality and ages of recruitment was examined using the mean annual total mortality rate observed in the park gray snapper stocks from 1974 to 1984 (i.e., Z = 1.29). Results for natural mortality rates (M) of are shown in Table 6. The resulting yields are peaked at low levels of natural mortality and asymptotic at higher levels. At all levels of natural mortality evaluated, increasing the age of recruitment greatly increases maximum yield with relatively little change in fishing effort (fishing mortality). At low levels of natural mortality, the expected yield drops sharply if F is increased past F MSY, the fishery mortality that produces maximum sustainable yield. At higher levels of M, F MSY shifts to the right and the stock is less easily overfished. In all cases, the expected yield increases rapidly from the onset of fishing and reaches a maximum at relatively low levels of fishing mortality. At the current age of recruitment W = 0.9) and estimated natural mortality rate (0.3), park gray snapper are growth-overfished. Reductions in fishing mortality of 80% will produce a 113% increase in yield (Table 6A). Given constant fishing mortality, increasing age at recruitment with a minimum size limit would increase yield 138 to 1,489% depending on the value ofm (Table 6B). At low levels of M, yield could be increased 1,489% by setting the size limit to 655 mm F.L. (26 in) to harvest MSY. At M = 0.4, setting the size limit at 322 mm F.L. (12.7 in) would increase yield by 138%. At M = 0.3, the size limit needed to achieve MSY equals 386 mm F.L. (15.2 in). Greater increases in yield of gray snapper can be achieved by raising age at recruitment through a minimum size limit than by reducing fishing mortality. Assuming natural mortality rate = 0.3, size limits of 254 or 305 mm F.L. (l0 or 12 in) and reductions in fishing mortality (80%) will achieve maximum sustainable yield and alleviate growth overfishing (Table 6). Virtual Population. -A Virtual Population Analysis (VPA) of mortality, recruit-

11 RUTHERFORD ET AL.: GRAY SNAPPER [49 Tab[e 5. Extended S [7 [59 5,056 5,704 17,28[ 3,248 9,044 17,073 9,843 38,693 34,96[ 27,648 26,791 36,174 34,26[ 40,800 38,563 [8,350 1[,432 22,605 23,766 21,984 2[,274 7,7[5 4,637 [,861 3,885 5,258 5,379 2,381 2, , ,87[ ,169 1,270 92,945 65,865 59,552 58,738 75,714 81,856 76,680 ment, and stock size was conducted using the annual estimates of total fish harvested by age classes shown in Table 5. Output of the VPA model was "tuned" by adjusting the terminal age-class fishing mortality rate to obtain the best correlation between age-specific fishing mortality rate and effort (standardized to recreational HPUE). The best relationship for gray snapper was significant (r 2 = 0.80, N = 30, P < 0.001) and was obtained using a terminal F value of Stock estimates by year and age class are shown in Table 7. Total numbers of gray snapper declined from 1974 (908,136) to 1982 (437,438), but have since increased slightly. Estimates for 1985 show a slight decline in stock, but are not reliable as final year model outputs. Recruitment (Fig. 9) appears to have declined sharply from 1976 to 1979, but has increased in recent years. There was no relationship between parent stock size (# mature females) and recruits although there was a positive trend (P < 0.10, N = 11). The percent of the total recruited population harvested ranged from 17 to 37% (Fig. 10). Population estimates of gray snapper showed poor correlation with standardized harvest rates (Fig. 11), possibly because the assumption of constant natural mortality used in the VPA may be false. Instantaneous fishing mortality rate estimates by age class for each year are shown in Table 8. The highest rates of fishing mortality occurred on ages 4 to 7 year old fish during most years. Average annual fishing mortality rates for the entire recruited stock (Fig. 12) were relatively constant from 1974 to 1976 ( ), but then increased markedly through 1979 to a high of This rate dropped to around 1.0 in 1980 and 1981 and then to 0.4 in A slight increase in fishing mortality was noted in Estimates of yearly average instantaneous total mortality rate (Z) were made from the VPA-generated fishing mortality rates by adding in the natural mortality rates for each age class and averaging all fully recruited classes for each year. Average annual total mortality rates from this approach were generally lower than those estimated from annual catch curves (Fig. 8). Both estimates follow a similar trend from 1974 to 1979, but then catch curve estimates become consistently higher than those estimated by the VPA. The VPA average fishing mortality rate, , for fully recruited stock is Assuming an average natural mortality rate of 0.3, the catch curve average fishing mortality rate equals A catchability coefficient (q = 5.60,10-6 ) was estimated by the VPA by regressing age-specific fishing mortality rates (F) on annual total standardized effort. DISCUSSION Previous reports have noted that gray snapper use the waters of Everglades NP as a "nursery area." Croker (1962) reported that anglers caught only immature

12 150 BULLETIN OF MARINE SCIENCE, VOL. 44, NO. I, 1989 Figure 9 (left). Estimated recruitment of gray snapper in the Aorida Bay area, Figure 10 (right). Percent of total recruited population harvested in the Aorida Bay area, gray snapper within the park averaging around 0.5 lb. Approximately 90% of the gray snapper he observed harvested were 2-3-yr-old fish, with very few fish over 4 years and none over 5. More recent studies in 1979 by Rutherford et al. (1983) confirmed that 2-3-yr-old fish still comprised most of the fishery. Rutherford et al. (1983) found fish up to 7 years of age harvested, but few over age 4. The predominance of fish less than 4 years old in the harvest supports conclusions of tagging studies (N. Funicelli, pers. comm.) that this species moves out of the park as it reaches maturity. This offshore movement is reflected to some extent by the slightly larger mean size offish harvested from Cape Sable, Shark River and other outer regions of the park's estuaries. The most abundant catches of gray snapper within the park have consistently occurred during the late summer. Higman (1967) recognized this seasonality and hypothesized that flushing of mangroves by rainfall, high temperature, and poor oxygen conditions within the upper reaches of the estuary caused gray snapper to congregate in channels and on outer banks during this period. Such congregations could increase their availability to fishermen. Although the harvest of gray snapper has been consistently highest during the season of highest runoff each year, high rainfall years do not correlate with high,.,.. N ~ n n ~ ~ ~ 81 U ~ ~ e + HPUE X lo-t Figure 11 (left). Population size and standardized harvest rates (X 106)of gray snapper in the Aorida Bay area, Figure 12 (right). Estimated average instantaneous fishing mortality of recruited gray snapper in the Aorida Bay area,

13 RUTHERFORD ET AL.: GRAY SNAPPER 151 Table 6. The effects of changing age at recruitment (t,) and fishing mortality rate (F) on yield-perrecruit (grams) of gray snapper in Everglades National Park. Four values of natural mortality rate are used. Woo= 8,370 gm, K = 0.101, to = -0.32, tm x= 18 yrs (Manooch and Matheson, 1983) A. Given 1.-. adjust F to get maximum sustainable yield (MSY) Yield at % Increase Current M t, F FMSY Current F FMSY F Yield III B. Given F, adjust \,. to get MSY for gray snapper Current Yield at % [ncrease M F \,. MSY Current\,. 1, MSY \,. Yield ,300 +1, gray snapper production. Higman (1967) reported an inverse relationship between catch rates and rainfall of the previous year. He believed that years with less than 50 in of rainfall produced conditions which favored survival and growth of juvenile gray snapper. Rutherford et al. (1983) also reported that fish caught from areas of higher salinity were larger at ages 1-4 years than fish from more brackish regions of the park. The harvest rate record now available suggests a 3-year lag between rainfall and catch rates; there is a significant inverse relationship between gray snapper catch rates and rainfall or water levels in the upland marshes (P-37 Station, Taylor Slough) 3 years before (r = 0.553, N = 21). This relationship may operate at least partially through recruitment as the recruitment level (no. age 1 fish) was inversely correlated with water levels (but not rainfall) in the Taylor Slough upland marsh the previous year (r = 0.709, N = 12). Annual estimates of the number of l-year-old fish within the park's population (within areas 1-5), based on the VPA analysis, declined from the mid-1970's to 1979 and then increased irrespective of high rainfall years. The mean size of gray snapper harvested does not support hypothesized increases in juvenile abundance following low rainfall years. An alternative explanation may lie in the movement of older fish. During low rainfall years, sub-adult fish may tend to remain within the estuaries longer under higher salinity conditions, thereby temporarily increasing populations and their availability to harvest by park fishermen. This alternative is supported to some extent by the fact that the size of gray snapper

14 152 BULLETIN OF MARINE SCIENCE, VOL. 44, NO. I, 1989 Table 7. Estimated total population by age of gray snapper in the Florida Bay area, Age 74 7S I 348, , , , , , , , , , , , , , , , , , ,481 78,293 90, ,016 80,823 79, ,928 42,332 20,373 33,143 23,965 19, ,436 8,876 20,604 7,407 9,689 7, ,258 3,971 3,274 6,237 1,233 2,137 Total 908, , , , , ,813 harvested was more influenced by changes in the number oflarger fish as opposed to numbers of small fish. We believe that the current harvest of gray snapper does not greatly impact the park's stock. Although the estimated total annual harvest declined nearly continuously from 1973 to 1983, only portions of this decline were due to reduced stock abundance. The major portion of the declining harvest through 1979 can be accounted for by reduced numbers of fishermen. Harvest rates increased during the 1970's, but total harvest continued to decline due to reduced effort. The number of fishermen has increased since 1979, but increased harvest and effort for spotted seatrout and red drum and bag limits have prevented any large increase in annual harvest of gray snapper. The linear relationship of annual total harvest to annual effort suggests that additional increases in harvest are possible. The lack of a clear relationship between annual effort and resulting stock abundance also suggests that immigration, recruitment, or perhaps varying natural mortality are more important influences on stock abundance than fishery harvest. Although the yield-per-recruit analysis indicated park gray snapper are presently growthoverfished, it is not known how much of the estimated fishing mortality is due to emigration of individuals out of the park as they mature. Gray snapper probably migrate out of the park to offshore reefs (N. Funicelli, pers. comm.) where they are subjected to an unknown level of fishing mortality. Offshore, gray snapper live to at least 21 years of age and experience lower mortality (Z = 0.60) than in the park (Manooch and Matheson, 1983). Although the location of the park's spawning stock is unknown, recent studies of larval and juvenile fish within the park suggest that reefs off the middle and lower Florida Keys probably supply much of recruitment to the park population (Rutherford et ai., 1989). Therefore, future management of the park's gray snapper population Table 8. Estimated instantaneous rate of fishing mortality by age for gray snapper in the Florida Bay area, Age 74 7S I Avg yrs

15 RUTHERFORD ET AL.: ORA Y SNAPPER 153 Table 7. Extended , , , , , , , , , , , ,534 91,514 82,051 99,783 83,685 82,677 92,792 58,753 35,]49 31,293 50,437 39,281 30,752 18,929 11,499 10,65 13,510 18,219 9,278 5,778 6,549 4,016 5,634 5,940 8, ,3]2 2,970 1,820 ],899 2, , , , , , ,838 must consider harvest occurring outside the park, particularly in respect to recruitment. We found a weak positive correlation between gray snapper parent stock size and recruitment. ACKNOWLEDGMENTS We greatly appreciate the efforts of D. Buker, R. Dawson, C. Spadaro, J. Heath, and B. Pettit for collecting and analyzing data used in this report, C. Doffermyre for typing the manuscript, and anonymous reviewers for significantly improving the manuscript. LITERATURE CITED Beaumariage, D. S Returns from the Schlitz tagging program including a cumulative analysis of previous results. Ra. Dept. Nat. Res. Lab., Tech. Ser pp. Croker, R. A Growth and food of the gray snapper, Lutjanus griseus. Everglades Nationa] Park. Treans. Amer. Fish. Soc. 91: Davis, G. E Changes in the Everglades National Park red drum and spotted seatrout fisheries, Fishing pressure, environmental stress on natural cycles. Pages in Proc. Colloquium on the Biology and Management of Red Drum and Seatrout. Gulf States Mar. Fish. Comm. Fox, Jr., W. W Program documentation for COHORT (a fortran IV computer program). NOAA/NMFS SWFC, Wolla, California. 20 pp. Higman, J. B Relationships between catch rates of sport fish and environmental conditions in Everglades National Park, Rorida. Proc. Gulf Carib. Fish. lnst. 19: Manooch, C. S. and R. H. Matheson Age, growth and mortality of gray snapper from Rorida waters. Proc. Ann. Conf. S.E. Assoc. Fish Wildlife Agencies. 35: 33 ]-344. Murphy, G. I. ] 965. A solution of the catch equation. J. Fish. Res. Bd., Canada. 22: Pauly, D On the interrelationships between natural mortality, growth parameters and mean environmental temperature in 175 fish stocks. J. du Conc. 39: Ricker, W. E Computation and interpretation of biological statistics offish populations. Bull. Fish Res. Bd. Can. 119 Ottawa Canada: Dept. Env't. Fish. Mar. Servo 382 pp. Table 8. Extended vg

16 154 BULLETINOFMARINESCIENCE,VOL.44, NO. I, 1989 Robson, D. S Estimation of the relative fishing power of individual ships. I.C.N.A.F. Res. Bull. (3): Rutherford, E. S., T. W. Schmidt and J. T. Tilmant Early life history of spotted seatrout Cynoscion nebulosus and gray snapper Lutjanus griseus in Florida Bay, Everglades National Park, Florida. Bull. Mar. Sci. 44: , E. B. Thue and D. G. Buker Population structure, food habits and spawning activity of gray snapper, Lutjanus griseus. in Everglades National Park. SFRC-83/02. Homestead, Florida: Everglades NP, South Florida Research Center. 41 pp. Tabb, D. c., D. L. Dubrow and R. B. Manning The ecology of northern Florida Bay and Adjacent estuaries. Fl. State Bd. Conserv. Tech. Ser pp. Tilmant, J. T A history and an overview of recent trends in the fisheries of Florida Bay. Bull. Mar. Sci. 44: DATEACCEPTED: August 10, ADDRESSES:(E.S.R., l.t.t., E.B.T., T. W.S.) South Florida Research Center, Everglades National Park, P.O. Box 279, Homestead, FL PRESENTADDRESS:(E.S.R.) University of Maryland, Chesapeake Biological Laboratory, Box 38, Solomons, MD