Chi-Lu Sun

High Value and Long Life—Double Jeopardy for Tunas and Billfishes

The first standardized, global assessment of these fishes, using Red List criteria, reveals threatened species needing protection. There is growing concern that in spite of the healthy status of several epipelagic (living near the surface) fish stocks (1), some scombrid (tunas, bonitos, mackerels, and Spanish mackerels) and billfish (swordfish and marlins) species are heavily overfished and that there is a lack of resolve to protect against overexploitation driven by high prices (2–5). Many populations are exploited by multinational fisheries whose regulation, from a political perspective, is exceedingly difficult. Thus, assessment and management is complicated and sometimes ineffective (4). Regional Fisheries Management Organizations (RFMOs) were created to manage and conserve scombrids and billfishes because of their transnational distributions and widespread economic importance (6). However, species-specific catch data for many scombrids and billfishes are not collected or are aggregated with other species. Even for the larger tunas, for which relatively rich data exist, population assessments and data are complex (1) and are difficult to combine across RFMOs, which prompts a need for alternative means of assessment.

Modelling the growth of crustacean species

Crustaceans play an important role in marine ecosystem and worldwide fisheries. Accurate and quantitative description of growth is crucial in modelling the demographics and fisheries stock assessment. The stepwise growth as a result of the moulting process and the lack of permanent calcified structures make the traditional approaches developed for finfish inappropriate for crustaceans. This study reviews the data collection, quantitative methods and various sources of uncertainty for modelling the growth of crustacean. The methods were reviewed based on their data requirement and mathematical/statistical complexity ranging from simple growth rate, continuous growth curve to stepwise growth curve and size transition matrix. A comparative example of different growth models was illustrated with four selected crustaceans: American lobster (Homarus americanus, Nephropidae), Dungeness crab (Metacarcinus magister, Cancridae), blue crab (Callinectes sapidus, Portunidae) and pronghorn spiny lobster (Panulirus penicillatus, Palinuridae). Model selection and multi-model inference based on the information theory was discussed for modelling the growth components of moutling increment and intermoult period. Various drivers in determination of the growth pattern of crustaceans were evaluated, including life history strategy and environmental stress. The incorporation of environmental factors into the development of growth models and the recent progress in age-determination were highlighted. We discussed possible research needs for better quantification of crustaceans growth.

Habitat suitability analysis and identification of potential fishing grounds for swordfish, Xiphias gladius, in the South Atlantic Ocean

Swordfish, Xiphias gladius, is a highly migratory species of important commercial value and widely distributed in three oceans. Recently, the South Atlantic swordfish captured as by-catch in longline fisheries targeting tunas has contributed greatly to the overall Atlantic swordfishs landing. In this study, we have developed a habitat suitability index (HSI) model to examine the relationships between their spatio-temporal distribution and environmental factors and to identify potential fishing grounds for the swordfish in the South Atlantic Ocean using the Taiwanese distant-water longline fishery data and remote-sensing oceanographic data for 1998–2007. All the environmental factors considered – sea surface temperature (SST), mixed layer depth (MLD), sea surface height anomaly (SSHA), chlorophyll-a concentration (CHA) and ocean bathymetry (BAH) – were highly significant with most of the catch-per-unit-effort (CPUE) variation explained by SST. The most optimum habitat (i.e. hotspot) was found in the areas with SSTs of 27–28°C, SSHAs ofu2009−0.05 to 0.05 m, CHAs of 0.1–0.2 mg m−3 and BAHs ofu2009−4000 tou2009−4500 m. The arithmetic mean model with five environmental variables was found to be the most appropriate according to the information theory based on the evaluation of different empirical HSI models in combination with different environmental factors. The bimonthly geographic information system maps of the predicted HSI values were cross-validated by the observed CPUE, suggesting that the model can be used as a tool for reliable prediction of potential fishing grounds. Because the distribution and relative abundance of swordfish are sufficiently heterogeneous in space and time, the output of this study could provide a scientific basis for time–area closures based management of this species.

Application of the sex-specific age-structured assessment method for swordfish, Xiphias gladius, in the North Pacific Ocean

Swordfish are known to be sexually dimorphic. However, previous assessments of the status of swordfish in the North Pacific Ocean have ignored his. A sex-specific age-structured assessment model was therefore constructed and fitted to catch, catch-rate and length–frequency data for the wordfish fisheries that operate in the North Pacific Ocean. Except if natural mortality is lower than its “best” estimate, the results indicate that the pawning stock biomass in 2002 was at a high fraction of its unfished level and that the fishing intensity in 2002 was less than FMSY. Therefore, the wordfish stock in the North Pacific Ocean appears to be relatively stable at the current level of exploitation. However, the results of the assessment odel are sensitive to the values for natural mortality and the steepness of the stock-recruitment relationship. Forward projections based on samples rom a Bayesian posterior distribution indicate that there is negligible risk of the stock dropping below 40% of the unfished spawning stock biomass f fleet-aggregated fishing intensity remains at the current level. However, the risk of population depletion is higher if natural mortality is lower han the “best” estimate. 2006 Elsevier B.V. All rights reserved.

Reproductive biology of female bigeye tuna Thunnus obesus in the western Pacific Ocean.

The reproductive biology of female bigeye tuna Thunnus obesus was assessed by examining 888 fish (ranging from 84·9 to 174·4u2009cm fork length, LF ) caught by Taiwanese offshore longliners in the western Pacific Ocean from November 1997 to November 1998 and November to December 1999 and 258 gonad samples from these fish. The overall sex ratio of the catch during the sampling differed significantly from 0·5, but males were predominant in sizes >140u2009cm LF . Reproductive activity (assessed by histology), a gonado-somatic index, and the size-frequency distributions of whole oocytes indicated that spawning occurred throughout the year and the major spawning season appeared to be from February to September. The estimated sizes at 50% maturity (LF50 ) of females was 102·85u2009cm (95% c.i.: 90·79-110·21u2009cm) and the smallest mature female was 99·7u2009cm LF . They are multiple spawners and oocytes develop asynchronously. The proportion of mature (0·63) and reproductively active (0·70) females with ovaries containing postovulatory follicles indicated that they spawn almost daily. Batch fecundity for 15 females with the most advanced oocytes (>730 µm) ranged from 0·84 to 8·56 million eggs (meanu2009±u2009s.d. = 3·06u2009±u20092·09). The relationships between batch fecundity (FB , in millions of eggs) and LF (cm) and round mass (MR , kg) were FB=9·91×10-14LF6·38 (r(2) u2009=u20090·84) and FB=8·89×10-4MR2·05 (r(2) u2009=u20090·80), respectively. The parameters estimated in this study are key information for stock assessments of T. obesus in the western Pacific Ocean and will contribute to the conservation and sustainable yield of this species.

Spatial and environmental determinants of the distribution of Striped Marlin (Kajikia audax) in the western and central North Pacific Ocean

Striped Marlin is a highly migratory species distributed throughout tropical and temperate waters in the North Pacific Ocean. The habitat characteristics of Striped Marlin in the western and central North Pacific Ocean were examined using generalized additive models by modelling fishery catch-rates as a function of remotely-sensed environmental covariates, including sea surface temperature (SST), chlorophyll-a concentration (CHL), mixed layer depth (MLD) and sea height anomalies (SHA). SST explained the largest proportion of the deviance, and is therefore considered the best predictor for the habitat of Striped Marlin. Spatial distributions of the relative density of Striped Marlin indicated that there is a seasonal north–south migration, and that the highest densities occur in the central North Pacific Ocean. The preferred habitat characteristics of Striped Marlin in high density areas were identified as SST between 23 and 26xa0°C, MLD within 30xa0m depth, and CHL around 0.08xa0mgxa0m−3, while no preferred range was found for SHA. The results of this study could improve our understanding of Striped Marlin spatial distributions and habitat characteristics.

Population dynamics and status of striped marlin (Kajikia audax) in the western and central northern Pacific Ocean

AnewunderstandingofthestructureofstripedmarlinstocksinthePacificOceanwasthebasisforestimating the population dynamics in the western and central northern Pacific (1975-2010). Dynamics were estimated using large- scale spatial data and a fully integrated length-based and age-structured model. The model used fishery-dependent catch, size composition and catch per unit effort (CPUE) as likelihood components. Time-varying selectivity patterns were used inthemodellingtoaccount forchangesinfishing practices.Estimatesoffishing intensityshowedapatternofexploitation generally exceeding the levels associated with maximum sustainable yield (MSY). Estimates of spawning biomass and recruitment described a population that was relatively stable near biomass levels associated with MSY until the 1990s, when recruitment declined and biomass levels fell below those associated with MSY. The reduction in recruitment could beexplainedbyalossofspawningbiomassandpotentiallychangesinenvironment.Thefutureprospectsofrebuildingthe stock will depend on the relative importance of the roles maternal biomass and environment play in determining recruitment strength. Additional keywords: environmental versus maternal effects, integrated modelling, time-varying selection.

Analyses of yield and spawning stock biomass per recruit for the South Atlantic albacore (Thunnus alalunga)

Abstract Yield per recruit and spawning stock biomass per recruit models were developed to study the interaction between the Taiwanese longline and South African baitboat fisheries for South Atlantic albacore. The Taiwanese longline fleet was found to have a logistically-distributed selectivity pattern with many age classes contributing significantly to the catch. The South African baitboat fleet, on the other hand, had a normally-distributed selectivity pattern with relatively few ages contributing to the catch. This fact profoundly influenced the response of yield and spawning stock biomass per recruit to different levels of fishing mortality exerted by each fleet. The results indicated that, in order to maximize total equilibrium yield per recruit to about 2.2xa0kg, the South African fishing mortality rate should probably be increased substantially whereas the Taiwanese fishing mortality rate is near optimal. In order to keep the spawning potential ratio above the 20% minimum suggested by Goodyear [Coll. Vol. Sci. Pap. 32 (2) (1989) 487], the Taiwanese apical fishing mortality rate should not exceed 0.55xa0yr −1 , regardless of the South African apical fishing mortality rate.

Sex-structured population dynamics of blue marlin Makaira nigricans in the Pacific Ocean

The population dynamics of the blue marlin Makaira nigricans stock in the Pacific Ocean were estimated for 1971–2011 using a fully integrated length-based, age-, and sex-structured model. Fishery-specific catch, size composition, and catch-per-unit of effort were used in the modeling as likelihood components. Estimated dynamics were consistent with a stock that is fully exploited and stable over the last several years. No significant trends in recruitment were noted; however, female blue marlin were estimated to make up a majority of the catch, and historical exploitation has disproportionately changed the age structure of females relative to males. This result is due to differences in assumed life history and estimated selectivity. Changes to important life history parameters that are responsible for the productivity of the stock would potentially change the interpretation of current stock status.

Regional variation in otolith Sr:Ca ratios of African longfinned eel Anguilla mossambica and mottled eel Anguilla marmorata: a challenge to the classic tool for reconstructing migratory histories of fishes

Otolith Sr:Ca ratios of the African longfinned eel Anguilla mossambica and giant mottled eel Anguilla marmorata from nine freshwater sites in four rivers of South Africa were analysed to reconstruct their migratory life histories between freshwater and saltwater habitats. For A. mossambica, the Sr:Ca ratios in the otolith edge differed significantly among rivers and had large effect sizes, but did not differ among sites within a river. Otolith Sr:Ca ratios did not differ among rivers for A. marmorata. When rivers were pooled, the edge Sr:Ca ratios of A. mossambica were not significantly different from those of A. marmorata. According to the river-specific critical Sr:Ca ratio distinguishing freshwater from saltwater residence, most A. mossambica and A. marmorata had saltwater habitat experience after settlement in fresh water. This was primarily during their elver stage or early in the yellow eel stage. During the middle and late yellow eel stage, freshwater residency was preferred and only sporadic visits were made to saltwater habitats. The data also suggest that regional variations in otolith Sr:Ca ratios affect the critical Sr:Ca value and are a challenge for the reconstruction of migratory life histories that should be explicitly considered to avoid bias and uncertainty.