Spawning dynamics and age structure of wild caught Tasmanian striped trumpeter during the 2010 spawning season. S.R. Tracey, G.E. Ewing & J.M.

Transcription

1 Spawning dynamics and age structure of wild caught Tasmanian striped trumpeter during the 2010 spawning season S.R. Tracey, G.E. Ewing & J.M. Lyle September 2011

2 Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 49, Hobart, Tasmania Ph. (03) Fax (03) The opinions expressed in this report are those of the author/s and are not necessarily those of the Institute for Marine and Antarctic Studies. Institute for Marine and Antarctic Studies, University of Tasmania 2011 Copyright protects this publication. Except for purposes permitted by the Copyright Act, reproduction by whatever means is prohibited without the prior written permission of the Institute for Marine and Antarctic Studies.

3 Executive summary This report summarises the size/age composition and reproductive dynamics of striped trumpeter sampled from five sites around Tasmania during the 2010 spawning season, August November. The project was designed to provide a spatially explicit assessment of the spawning dynamics of striped trumpeter in relation to current management strategies that include a spawning season closure and minimum size limits. After an extended period of poor recruitment, this study revealed evidence of a relatively strong recruitment of fish spawned in 2004 or 2005, depending on age estimation variability. This cohort(s) was identified across all regions, but was particularly evident in samples collected along the east coast. On the west coast, and to a lesser extent the south coast, larger fish across a broad size and age range were still caught. When considering the size distribution of fish caught sampled in relation to the current legal minimum size limit of 500 mm TL only a small proportion of fish were undersized (0 11% depending on region). Anecdotal reports suggest that the cohort of newly recruited fish was first detected by fishers using hook methods in 2009, and it has become more evident in catches during Interestingly, this cohort was not obvious from inshore net catches, noting that the especially strong 1993 cohort was caught in great numbers by inshore net fishers. Intensive sampling for striped trumpeter confirmed that the spawning season extends over three months in the late austral winter early spring period, with peak spawning activity occurring in September and October. No significant latitudinal variation in the onset of the spawning season was detected. There was however, a regional trend in the duration of the spawning season, with the season more protracted moving counter-clockwise around Tasmania, from the northeast coast to the west coast. The confirmation of the timing of spawning and the fact that most fish off the northeast and east coasts had finished spawning by November indicates that the current spawning season (1 st September to the 31 st October) covers at least two-thirds of the spawning season, including the period of peak spawning activity. The size at 50% maturity of females (equivalent to 603 mm TL) calculated from the fish collected during 2010 was within 8mm of previous estimates. The high level of agreement between estimates suggests that this estimate is robust and is over 100 mm greater than the current legal minimum size limit of 500 mm TL. Main findings The timing of the spawning season closure is appropriate to provide protection to the spawning stock of striped trumpeter in Tasmania. There is scope to re-assess the minimum legal size limit based on predicted size at 50% maturity. Currently fish are harvested at a size approximately 100 mm smaller than size at 50% maturity. A new recruitment pulse has entered the fishery which is a positive sign given the apparent lack of recruitment over the last 14 years. IMAS Report Page 3

4 Contents Executive summary... 3 Main findings... 3 Introduction... 5 Methods... 7 Sample collection... 7 Biological sampling... 7 Size at maturity... 7 Results... 9 Fork length - total length relationship... 9 Size and age composition by region... 9 Size and age at maturity Timing of the 2010 spawning season Discussion Size/Age composition Spawning dynamics Acknowledgments References IMAS Report Page 4

5 Introduction Striped trumpeter (Latris lineata) is a large temperate reef species that is relatively long lived, attaining a maximum age of over 40 years (Tracey & Lyle, 2005) and growing to 25 kg in weight (Gomon et al., 2008). The species is widely distributed around the southern hemisphere, found in all the major oceanic basins (Tracey et al., 2006), but geographically separated populations are believed to be genetically distinct (Tracey et al. 2007a). The species has had a long history of commercial and recreational exploitation in Tasmania, targeted for its high quality large white fillets. The species is taken by a variety of fishing methods, with hooks and gillnets being the primary methods. Juvenile striped trumpeter are taken predominantly by graball net in inshore waters and usually in depths <50 m, whereas adult fish are taken in deeper offshore waters by hook methods (dropline, handline, bottom longline, trotline) and as by-product in large mesh gillnets (shark nets). Catches are concentrated off the east coast, including Flinders Island, as well as off the south and south-west coasts of Tasmania. Limited catches are taken off the west coast (Hartmann & Lyle, 2011). Commercial production of striped trumpeter (south of S) in the early 1990s was over 110 tonnes per annum. By 2000/01, catches had almost halved to less than 50 tonnes and have remained low since that time. The reported catch of 12.8 tonnes for 2009/10 is a slight decline from 13.3 tonnes in 2008/09 and represents the lowest catch reported since the mid- 1980s (Hartmann & Lyle, 2011). A survey of the recreational fishery during 2000/01 provided an estimated annual recreational harvest of 38 tonnes, which was only slightly less than the commercial catch for the same period (Lyle, 2005). Estimates of recreational catches in 2007/08 were highly uncertain but likely to be lower, with an estimated 19 tonnes for combined catches of striped and bastard trumpeter (Lyle et al. 2009). These estimates however, do not fully represent catches by charter boats, many of who target striped trumpeter. Marked recruitment variability appears to be a feature of striped trumpeter, and although the species is long-lived, prolonged periods of poor recruitment combined with the impacts of fishing and natural mortality have the capacity to severely deplete the size of the mature adult stock. In August 2000, the State Government introduced a combined 250 kg trip limit for striped trumpeter, yellowtail kingfish and pink snapper for all commercial fishers (Commonwealth and State) in inshore and offshore waters relevant to Tasmania. This measure was introduced to limit the potential for expansion of commercial effort directed at these species. A daily bag limit of five and possession limit of eight striped trumpeter was also introduced for recreational fishers. The legal minimum size limit for striped trumpeter was raised from 35 to 45 cm total length (TL) in November 2004 in recognition that the smaller size limit was substantially below the IMAS Report Page 5

6 size at maturity reported at that time. The recreational bag limit was also dropped and effectively replaced by the possession limit of eight fish. Taking effect in November 2009, the minimum size limit was increased to 50 cm TL and a recreational on water bag limit of 4 striped trumpeter was introduced based on concerns over the stocks, exacerbated by an apparent extended period of poor recruitment. Furthermore, a two month spawning closure (September-October) was implemented for the first time in 2009 and has recently been extended for 3 more years to 2013 (inclusive). The latest management interventions for striped trumpeter are based around the spawning dynamics of the species, in particular the spawning season closure and minimum legal size limit. The aim of this study is to provide information on the size at maturity and timing of the striped trumpeter spawning season and size at maturity to assist resource managers to optimise, if necessary, the current closed seasons and size limits for striped trumpeter. IMAS Report Page 6

7 Methods Sample collection Striped trumpeter were collected from commercial and recreational charter vessels from July to November 2010 inclusive. Sampling regions are outlined in Table 1, with successful trips completed from all sites for each month with the exception of the east and west coast sites in July. All fish were caught using baited hook and line. On each trip, if a suitable sample size (approximately 20 fish) were collected subsequent fish caught were tagged and released unless they were significantly larger or smaller than the modal length of fish that were kept. Table 1. The number of fish sampled from the five sampling regions around Tasmania. Total biological represents the number of fish harvested and subsequently processed to provide a suite of biological traits. Total T/R is the number of fish tagged and released. Region Depth range (m) Days fished Total sampled Total biological Total T/R Northeast coast (Banks strait) East coast Southeast coast South coast West coast Biological sampling Fork length and total length ( 1 mm), sex, total weight ( 10 grams), gonad weight ( 1 gram) and macroscopic maturity stage (based on a seven stage criteria for females and a five stage criteria for males, defined by Tracey et al., 2007b) were recorded. Gonadosomatic index (GSI) was calculated from each female was estimated as: GW GSI 100 TW GW where GW is gonad weight and TW is total weight. For analyses on GSI values calculated for females greater than 542 mm FL (size at 50% maturity as estimated from individuals sampled in 2010) are reported. Size at maturity A logistic regression was fitted to the maturity-at-length data (pooled across region) of fish collected throughout the spawning season (September November). Maturity was described in a binary configuration (0 = immature, stages I III for females; and, 1 = mature, stages IV VII for females. The data were pooled into 20 mm categories. The logistic model fitted to the data was e P 1 e o FL 1 o FL 1, IMAS Report Page 7

8 where P represents the probability of being mature at size FL, and β 0 and β 1 are the regression coefficients for the intercept and fork length, respectively. The model fit was estimated by maximising the log-likelihood of the binomially distributed error term. The length where 50% of females were determined to be mature (L 50 ) was derived from the equation L 50 o. 1 Age at 50% maturity (t 50 ) was calculated using the estimate of length at 50% maturity and the von Bertalanffy growth parameters based on the single phase growth model reported in Tracey and Lyle (2005). The von Bertalanffy growth function (VBGF) rearranged to the form: 1 50 t ln k t o FL FL FL where t 50 is the age at 50% maturity, FL is the asymptotic fork length in mm, k is the Brody growth parameter, and t 0 is the hypothetical age at zero length. Tracey & Lyle (2005) found no significant difference in the von Bertalanffy growth functions (VBGF) describing the growth of males and females, hence, in the present study the parameters of the VBGF used were of the pooled VBGF, estimated as FL = mm, k = 0.15 yr and t 0 = years. 50 IMAS Report Page 8

9 Results Fork length - total length relationship A significant linear relationship was reported between the fork length and total length of striped trumpeter (TL = 1.08 x FL , r 2 = 0.99, p < ). This relationship estimates that at the current minimum legal size limit of 500 mm TL the corresponding fork length is 456 mm. Fig. 1. The relationship between striped trumpeter fork length and total length, fitted with a linear regression function. Size and age composition by region The modal length from all sites ranged from mm FL (Fig. 2), this corresponds to an age of 5-6 years based on interpretation of the incremental structure of otoliths. The distribution of this mode class across all regions suggests that a successful recruitment event occurred in 2004 or 2005 depending on the variability in individual growth rates (Fig. 3). Fish from a particularly strong year class spawned in 1993, which dominated catches from the mid-1990s to late in the first decade of the 21 st century, was still evident in the south and west coast samples, although now relatively insignificant compared to the more recent recruitment event(s). Interestingly these two regions were where a greater proportion of larger fish were caught compared with the northeast, east and southeast sampling regions. IMAS Report Page 9

10 Northeast coast Striped trumpeter caught from the northeast coast of Tasmania ranged in length from mm FL. The average size of fish caught was 511 mm FL, and the modal fork length was 485 mm (Fig. 2). By converting these fork length measurements to total length using the regression presented in Fig. 1, it was calculated that 11% of the 155 fish caught from the northeast were sub-legal (< 500 mm TL). A total of 5% of the fish caught were above 600 mm FL. East coast Striped trumpeter caught from the east coast of Tasmania ranged in length from mm FL. The average size of fish caught was 533 mm FL, and the modal fork length was 515 mm (Fig. 2). By converting these fork length measurements to total length using the regression presented in Fig. 1, it was calculated that 10% of the 124 fish caught from the east were sub-legal (< 500 mm TL). A total 18% of the fish caught were above 600 mm FL. Southeast coast Striped trumpeter caught from the southeast coast of Tasmania ranged in length from mm FL. The average size of fish caught was 508 mm FL, and the modal fork length was 500 mm (Fig. 2). By converting these fork length measurements to total length using the regression presented in Fig. 1, it was calculated that 10% of the 81 fish caught from the northeast were sub-legal (< 500 mm TL). A total of 4% of the fish caught were above 600 mm FL. South coast Striped trumpeter caught from the south coast of Tasmania ranged in length from mm FL. The average size of fish caught was 550 mm FL, and the modal fork length was 510 mm (Fig. 2). By converting these fork length measurements to total length using the regression presented in Fig. 1, it was calculated that less than 1% of the 152 fish caught from the south coast were sub-legal (< 500 mm TL). A total of 15% of the fish caught were above 600 mm FL. West coast Striped trumpeter caught from the west coast of Tasmania ranged in length from mm FL. The average size of fish caught was 629 mm FL, and the modal fork length was 520 mm (Fig. 2). By converting these fork length measurements to total length using the regression presented in Fig. 1, it was calculated that 1% of the 88 fish caught from the west coast were sub-legal (< 500 mm TL). A total of 49% of the fish caught were above 600 mm FL. IMAS Report Page 10

11 Fig. 2. Size frequency composition of striped trumpeter caught between July and November 2010 from five regions around Tasmania. % < legal indicates the percentage of catch that was below the minimum legal size limit of 500 mm TL, converted to fork length based on the regression in Fig. 1. IMAS Report Page 11

12 Proportion Spawning dynamics of striped trumpeter northeast coast n = east coast n = southeast coast n = south coast n = west coast n = 76 0 Birthyear Fig. 3. Year-class composition of striped trumpeter caught between July and November 2010 from five regions around Tasmania. Note not all fish were aged due to indistinguishable increment structure within some otoliths. IMAS Report Page 12

13 Size and age at maturity The size at 50% maturity of female striped trumpeter sampled during 2010 was 551mm FL (603mm TL). This was similar to the estimate (543mm FL) presented by Tracey et al. (2007b), who pooled data collected opportunistically over a 14 year period (Table 2). The modal size composition collected during 2010 straddled the predicted size at maturity. This provided a more knife-edge fit for the logistic regression (Fig. 4). This meant that the size at 25 and 75% maturity were refined, with the former increasing by 19 mm and the later decreasing by 13 mm from the previously reported estimates (Table 2). Table 2. Estimated lengths of females at 25, 50 and 75% maturity from samples collected in 2010 and for comparison estimates calculated from data pooled over 14 years from In both cases samples were constrained to individuals caught over the spawning period (August November) estimate pooled estimate Proportion mature Length (mm) Age (years) Length (mm) Age (years) L L L Fig. 4. Proportion of mature females by 20 mm length intervals during the 2010 striped trumpeter spawning season (August November), n = 168 (62 mature). The points were fitted with a logistic spline illustrating the optimal logistic maturation ogive (red spline). To provide a comparison the logistic maturation ogive calculated from fish caught during spawning seasons from is also shown, this latter curve was based on 120 (85 mature) individuals. IMAS Report Page 13

14 GSI Spawning dynamics of striped trumpeter Timing of the 2010 spawning season The mean gonadomsomatic index (GSI) of female striped trumpeter (above the size at 50% maturity) increased sharply from July to August. The index then remained high for the subsequent months of September and October before decreasing again in November (Fig. 5). This result suggests that female striped trumpeter were conditioned for spawning for the months of August to September and that the spawning season was essentially completed by November Jul/Aug Aug/Sep Sep/Oct Oct/Nov Nov/Dec Month Fig. 5. The mean gonadosomatic indices (GSI) estimated for females for each month (midmonth to mid-month) period from mid-july to mid-december inclusive, pooled across all sampling regions. Error bars represent the standard error of the mean. Numbers above the estimate indicate the number of individuals included in the average for the given month. Spatial variation in the timing of the start of the spawning season The average GSI of females larger than 542 mm FL (size at 50% maturity of females during 2010) from all regions in late August (23 rd 30 th ) were indicative that the fish were in spawning condition, with the exception of the southeast coast where the GSI was reported at a level that would indicate that the fish sampled was not in spawning condition (Fig. 6). These results suggest that for all sites, with the exception of the southeast, there is a low degree of spatial variability in the timing of the onset of spawning. It would appear that the onset of spawning is not influenced systematically by the latitude of the region as the individuals caught from the south coast were in spawning condition based on the average GSI, although the standard error from this region suggest that there was a degree of individual variability in spawning condition which would be expected early in the spawning season (Fig. 6). IMAS Report Page 14

15 Fig. 6. The average gonadosomatic indices (GSI) estimated for females ( ) sampled in late August by each sampling region. Error bars represent the standard error of the mean. Numbers above the estimate indicate the number of individuals included in the average. Spatial variation in the timing of the end of the spawning season By late October (19 th 27 th ) it was assumed that all individuals capable of developing to a viable reproductive state would have done so. Therefore, to assess spatial variability in the timing of the end of the 2010 striped trumpeter spawning season, we have assessed the proportion of females with a stage VII gonad (spent) against all females with a maturity stage greater than three (assessed as individuals contributing to the spawning biomass). This allowed for a greater sample size for each region than if we were to assess GSI of those individuals that were of a size greater than the reported size at 50% maturity. Although the sample sizes were still relatively low a latitiudinal trend (or possibly anti-clockwise trend) in the percentage of spent females (stage VII) was evident. All individuals from the northeast coast were spent by the end of October, this proportion decreased for the east and southeast coasts to 82 and 38% respectively. All females sampled from the south coast were still in a reproductively active state (stage IV VI), with none identified as spent, indicating a more protracted spawning season in that region. The sample size from the west coast was greater than for the other regions and of the sixteen females that were reproductively mature, only one was identified as spent, again suggesting a protracted spawning season in this area (Fig. 7). IMAS Report Page 15

16 100 Percent of females with stage 7 gonads NE E SE S W Region Fig. 7. The percentage of females with stage 7 gonads (spent) from all those with a gonad stage greater than or equal to IV (contributing to spawning) during the month of October by each sampling region. Numbers shown indicate the total number of individuals included from each region. IMAS Report Page 16

17 Discussion Size/Age composition After an extended period of poor recruitment, this study revealed evidence of a relatively strong recruitment event based on year classes estimated to have been spawned in 2004 and/or 2005 depending on age estimation variability 1. This cohort(s) was identified across all regions, but was particularly evident in samples collected along the east coast where it dominated the samples. On the west coast, and to a lesser extent the south coast, a broad size and age range of fish were present. Anecdotal reports suggest that this cohort was first detected by fishers using hook methods in 2009, although it became more evident during Interestingly this cohort was not obvious from inshore net catches, noting that the 1993 cohort was caught in great numbers by inshore net fishers. When considering the current legal minimum size limit of 500mm TL in relation to the size distribution of fish caught during 2010, only a small proportion of catch was undersize (0 11% depending on region). Spawning dynamics Striped trumpeter exhibit a discrete annual spawning cycle that extends over three months in the late austral winter early spring period, with peak spawning activity occurring in September and October. The timing of this spawning period was confirmed with the intensive (for this species) regional sampling conducted in Spawning is widespread throughout the distribution range around Tasmania, with females in spawning condition collected from all offshore sampling locations. Furlani and Ruwald (1999) suggested that spawning commenced and finished earlier at higher latitudes off Tasmania, although their study did not present empirical data to support this claim. The results from the current study provide little evidence for a significant latitudinal variation in the onset of the spawning season. The data from the 2010 sampling did, however, suggest that the season is more protracted moving counter-clockwise around Tasmania, from the northeast coast to the west coast. This may be related to temperature with the waters on the east, and particularly the northeast coast, warmer than the south and west coasts due to the intrusion of the east Australian current during the spring and summer months. The confirmation of the timing of the spawning season and the fact that most fished have finished spawning by November would indicate that the current spawning season closure from the 1 st September to the 31 st October covers at least two-thirds of the spawning season, including the period of peak spawning. The size at 50% maturity of females calculated from the fish collected during 2010 was within 8 mm of the estimate presented by Tracey et al. (2007b). The high level of agreement 1 There is some uncertainty surrounding the attributed ages to individuals because of the timing of sample collection (close to birth date) which means some individuals may be under or over aged by one year, depending on the structure of the otolith margin. IMAS Report Page 17

18 between these two estimates suggests that this estimate is robust. Using the regression model to convert fork length to total length, the size at 50% maturity for females is estimated at 603 mm indicating that the current minimum size limit is around 100mm smaller than the size at 50% maturity. Confirmation of this suggests that the minimum size limit may need to be reviewed if a management objective is to provide some protection to immature fish and provide individuals the opportunity to spawn at least once prior to recruiting to the fishery. Acknowledgments We thank the skippers and crew of the vessels chartered for sample collection and IMAS staff and volunteers who assisted with the fish processing. This study was conducted with funding from the Tasmanian Fishwise Community Grant Scheme, Project No. 2009/110. IMAS Report Page 18

19 References Furlani, D.M., Ruwald, F.P Egg and larval development of laboratory-reared striped trumpeter Latris lineata (Forster in Bloch and Schneider 1801) (Percoidei: Latridiidae) from Tasmanian waters. New Zealand Journal of Marine and Freshwater Research 33, Gomon, M.F., Glover, J.C.M., and Kuiter, R.H The Fishes of Australia s Southern Coast. Reed New Holland, Australia. Hartmann, K. and Lyle, J.M Tasmanian scalefish fishery 2009/10. Institute for Marine and Antarctic Studies. Fishery Assessment Report. Lyle, J.M /01 survey of recreational fishing in Tasmania. Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Technical Report 24. Lyle, J.M., Tracey, S.R., Stark, K.E., and Wotherspoon, S /08 survey of recreational fishing in Tasmania. Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Technical Report. Tracey, S.R., Lyle, J.M Age validation, growth modelling, and mortality estimates for striped trumpeter (Latris lineata) from southeastern Australia: making the most of patchy data. Fishery Bulletin 103, Tracey, S.R., Lyle, J.M. Duhamel, G Application of elliptical Fourier analysis of otolith form as a tool for stock identification. Fisheries Research 77, Tracey, S., Smolenski, A. and Lyle, J. 2007a. Genetic structuring of Latris lineata at localized and transoceanic scales. Marine Biology 152: Tracey, S.R., Lyle, J.M. and Haddon, M. 2007b. Reproductive biology and per-recruit analyses of striped trumpeter (Latris lineata) from Tasmania, Australia: Implications for management. Fisheries Research 84: IMAS Report Page 19