Search

Context: Within the western and central North Pacific Ocean stock of striped marlin (Kajikia audax), reproductive information is lacking for the central North Pacific. Aims: To derive estimates of sex-specific functional maturity ogives, eye-to-fork length at 50% maturity, reproductive phase composition, spawning season and sex ratios by sampling the Hawaii-based pelagic longline fishery. Methods: Gonad histology was exclusively used to confirm sex and estimate all reproductive life history traits. Key results: Females spawn in May–July; males are capable of spawning year-round. Best length at 50% maturity estimates were based on datasets restricted to the female spawning season (eye–fork length for females 152.2 cm and for males 109.0 cm). Length distributions and sex ratios are seasonally dynamic around Hawaii and differ from those in other Pacific regions. Conclusions: Length-at-maturity estimates for the central North Pacific are lower than all sex-specific estimates reported from other Pacific regions. Implications: The new maturity estimates will improve determinations of stock resilience and productivity in upcoming international assessments. Striped marlin spawning in the central North Pacific was previously little studied. Our investigation has shown that both females and males mature at smaller sizes than in all other Pacific regions surveyed. These results have direct application for current efforts to evaluate the health and resilience of the population in the western and central North Pacific, which is currently considered to be commercially overfished and subject to continued overfishing. [ABSTRACT FROM AUTHOR]

Striped marlin, Kajikia audax, have been in overfished condition in the Western and Central North Pacific, and overfishing is still occurring, prompting an urgent need to devise conservation and management measures based on the best, current information on biology and ecology of this species. Despite conventional tagging efforts off Hawaii, striped marlin movements are not known across the broader Pacific, and little research has been conducted since 2005. To address this gap, 31 popup satellite archival tags (PSAT) were deployed on striped marlin (138-192 cm eye fork length) between 2016 and 2019 via the Hawaii-based longline fleet. To complement tagging efforts, 148 fin clips were also collected for genetic analyses during June-August 2017. Tag attachments ranged from 1-365 days (median = 74), where mechanical failures and non-reporting tags lowered expected data returns. Striped marlin tracks revealed extensive spatial use of the Central Pacific, spanning 15°S to 43°N and 122 to 170°W, showing diverse seasonal dispersal patterns and individual movements, and some coincided in time and space with known spawning grounds. Genetic profiles of 55 Hawaii-landed striped marlin were assigned to two genetic groups: Australia, New Zealand and Hawaii (n = 19), and Hawaii alone (n=36), suggesting the Hawaii-based longline fleet interacted with individuals from multiple populations.A year-long track confirmed migration between the Central North Pacific and Australia (>9,400 km), and combined with genetic results, is the first to document regional connectivity. By combining tools from conventional and electronic tags, biological sampling, and genomic techniques, a more holistic understanding emerges, suggesting that striped marlin should be collectively managed. Under global warming scenarios and changing pelagic ecosystems, integrative knowledge is critical for designing effective management strategies for rebuilding sustainable populations across the Pacific Ocean.

Uncertainty regarding the movement and population exchange of Atlantic bluefin tuna (Thunnus thynnus) from the two primary spawning areas (Gulf of Mexico, Mediterranean Sea) is increasingly implicated as a major impediment for the conservation of this species. Here, two mixture methods were applied to natural chemical markers (δ18O and δ13C) in otoliths (ear stones) to comprehensively investigate the nature and degree of transoceanic movement and mixing of eastern and western populations in several areas of the North Atlantic Ocean that potentially represent mixing hotspots. Areas investigated occurred on both sides of the 45°W management boundary as well as waters off the coast of Africa (Morocco, Canary Islands) where both populations are known to occur. Projections of population composition (i.e., natal or nursery origin) from a multinomial logistic regression (MLR) classification method with different probability thresholds were generally in agreement with maximum likelihood estimates from the commonly used mixed-population program HISEA; however, predicted contributions for the less abundant population were occasionally higher for MLR estimates. Both MLR and HISEA clearly showed that mixing of Atlantic bluefin tuna in the Central North Atlantic Ocean was highly variable from year to year with expatriates of eastern or western origin commonly crossing into the other management area. Pronounced transoceanic movement and mixing of western migrants was also present off the coast of Africa, with the occurrence of western migrants in the Canary Islands and Morocco ranging from zero to the majority of the individuals assayed for the years examined. Results indicate highly variable rates of movement and population exchange for Atlantic bluefin tuna, highlighting the need for temporally resolved estimates of natal origin in mixing hotspots to improve population models used to evaluate the status of this threatened species.

The greater amberjack (Seriola dumerili) is a commercially and recreationally important marine fish species in the southeastern United States, where it has been historically managed as two non-mixing stocks (Gulf of Mexico and Atlantic). Mark-recapture studies and analysis of mitochondrial DNA have suggested the two stocks are demographically independent; however, little is currently known about when and where spawning occurs in Gulf of Mexico amberjack, and whether stock mixture occurs on breeding grounds. The primary objective of this study was to quantify stock mixture among breeding populations of amberjack collected from the Atlantic and Gulf of Mexico. Genetic data based on 11 loci identified very low, though statistically significant differentiation among Gulf of Mexico samples (GST = 0.007, GST′ = 0.009; all P = 0.001) and between reproductive adults collected from two spawning areas (GST = 0.007, GST′ = 0.014; all P = 0.001). Naïve Bayesian mixture analysis supported a single genetic cluster [p(S|data) = 0.734] whereas trained clustering (using Atlantic and Gulf spawning fish) gave the highest support to a two-cluster model (p(S|data) = 1.0). Our results support the argument that the genetic structuring of greater amberjack is more complex than the previously assumed two, non-mixing stock model. Although our data provide evidence of limited population structure, we argue in favour of non-panmixia among reproductive fish collected from the Gulf of Mexico and Florida Keys. [ABSTRACT FROM AUTHOR]

Summary: Two cod stocks (western Baltic cod, WBC, and eastern Baltic cod, EBC) are managed in the Baltic Sea which is characterized by different main spawning areas and different main spawning periods. In this study we analyse the spatial and temporal occurrence of spawning individuals of both cod stocks in the main spawning grounds of the Baltic Sea based on eight microsatellite loci. Our results suggest that EBC (Gadus morhua callarias) has formed currently temporally stable, substantially homogeneous population not only in the Bornholm Sea (ICES SD: 25) but also in the Arkona Sea (ICES SD: 24). The presented analyses proved that EBC (G. m. callarias) can temporarily also spawn in the Belt Sea. [ABSTRACT FROM AUTHOR]

Over the past 40 years, northwestern Atlantic bluefin tuna (Thunnus thynnus) have experienced high rates of exploitation, targeted fishing on the largest size classes of the population, and an unknown degree of Mediterranean-stock contribution. Lack of recovery despite targeted rebuilding efforts by management prompted an evaluation of the population for changes in age-structure, size-at-age, and stock mixing over three samples (1974-8, 1996-2002, and 2009-14) coinciding with a cycle of exploitation that initially targeted smaller fish, but then showed strong selectivity for the largest and oldest members of the population. Ages and mixing levels were estimated using port- and observersampled otoliths collected by the US National Marine Fisheries Service. A comparison of age structure between the 1970s sample and two later samples indicated strong age truncation had occurred, where both mean age (13, 4, and 7 years) and mean length (191, 166, and 159 cm curved fork length) declined among samples. In addition, minor changes in size-at-age were detected among the three samples. Otolith stable isotope analysis indicated fluctuating stock composition, with a substantially higher contribution of Mediterranean-origin fish in the 1990s (48% eastern stock contribution) than in the 1970s (0% contribution) and the most recent sample (4% contribution). Higher mixing and severe age truncation in the 1990s indicated that the northwestern Atlantic population was at a depressed state. Reduced mixing and a slightly expanded age structure in the most recent sample could suggest that recovery has begun. Still, to evaluate the hypothesized cycle of collapse and modest recovery more rigorously, an integrative assessment framework is needed to consider the dynamic nature of stock productivity, trans-oceanic migrations, and fishing selectivity. [ABSTRACT FROM AUTHOR]

Natural chemical markers (stable isotopes and trace elements) in otoliths of bigeye tuna ( Thunnus obesus) and yellowfin tuna ( T. albacares) were used to investigate their origin and spatial histories in the western and central Pacific Ocean ( WCPO). Otolith chemistry of young-of-the-year ( YOY) T. obesus and T. albacares from four regions in the WCPO was first determined and used to establish baseline chemical signatures for each region. Spatial variation in stable isotope ratios of YOY T. obesus and T. albacares was detected, with the most noticeable difference being depleted otolith δ18O values for both species from the far west equatorial and west equatorial regions relative to the central equatorial and Hawaii regions. Elemental ratios in otoliths were also quantified for YOY T. obesus and T. albacares collected in 2008, and several showed promise for distinguishing YOY T. obesus (Mg:Ca, Mn:Ca, and Ba:Ca) and T. albacares (Li:Ca and Sr:Ca). The natal origin of age-1 to age-2+ T. obesus and T. albacares was then determined for two regions of the WCPO, and mixed-stock analysis indicated that T. obesus and T. albacares in our west equatorial sample were almost entirely from local production, with a minor contribution from central equatorial waters. Similarly, T. albacares collected in Hawaii were exclusively from local sources; however, a large fraction of T. obesus in Hawaii were classified to the central equatorial region, suggesting that the movement of migrants from outside production zones (i.e., south of Hawaii) are important to Hawaii's domestic fishery. [ABSTRACT FROM AUTHOR]

In the Baltic Sea, two genetically distinct cod populations occur, the eastern and the western Baltic cod. Since 2006, cod abundance has increased substantially in the Arkona Basin (SD 24), the potential mixing area between the two stocks management areas, presumably due to spill-over from the eastern stock. In this study, the spatio-temporal dynamics of stock mixing were analysed using shape analysis of archived otoliths. Further, the impact of eastern cod immigration on recruitment in the western Baltic Sea was investigated using hydrographic drift modelling. The percentage of eastern Baltic cod in the Arkona Basin increased from ca. 30% before 2005 to .80% in recent years. Geographic patterns in stock mixing with a pronounced east-west trend suggest that immigration occurs north of Bornholm, but propagates throughout the Arkona Basin. The immigration cannot be attributed to spawning migration, as no seasonal trend in stock mixing was observed. Based on environmental threshold levels for egg survival and time-series of hydrography data, the habitat suitable for successful spawning of eastern cod was estimated to range between 20 and 50% of the maximum possible habitat size, limited by primarily low salinity. Best conditions occurred irregularly in May-end June, interspersed with years where successful spawning was virtually impossible. Using a coupled hydrodynamic modelling and particle-tracking approach, the drift and survival of drifters representing eastern cod eggs was estimated. On average, 19%of the drifters in the Arkona Basin survive to the end of the yolk-sac stage, with mortality primarily after bottom contact due to low salinity. The general drift direction of the surviving larvae was towards the east. Therefore, it is the immigration of eastern cod, rather than larval transport, that contributes to cod recruitment in the western Baltic Sea. [ABSTRACT FROM AUTHOR]

Atlantic cod (Gadus morhua) in the Northwest Atlantic off New England and southern Atlantic Canada exhibit a complex population structure. This region has three independently assessed stocks [Georges Bank, Gulf of Maine (GOM), and the 4X stock], all of which are known to mix with each other. Assessments of these stocks, however, assume no interpopulation mixing. Using simulations, we evaluated impacts of ignoring mixing resulting from seasonal migrations on the GOM assessment. The dynamics of the three stocks were simulated according to different scenarios of interstock mixing, and a statistical catch-at-age stock assessment model was fitted to the simulated GOM data with and without mixing. The results suggest that, while mixing causes measurable bias in the assessment, under the conditions tested, this model still performed well. Of the bias that does exist, spawning-stock biomass estimates are relatively sensitive to mixing compared with estimates of recruitment and exploitation rate. The relative timing of seasonal migration of the three stocks plays a critical role in determining the magnitude of bias. The scale and trends among years in the bias were driven by how representative the catch and survey data were for the GOM stock; this representation changed with the mixing rates. [ABSTRACT FROM PUBLISHER]

Cod in the Baltic Sea is assessed and managed as two separate stocks, i.e. eastern and western Baltic cod. The eastern Baltic cod has recently started to recover after several decades of severe depletion. In the present study, we suggest that the recovery of the eastern Baltic cod population has also substantially increased cod abundance in a specific area of the adjacent western Baltic management unit. This is investigated through long time-series of spatially resolved stock assessment data supplemented by genetic analyses of origin of the cod currently found in the transition area between the two populations. Due to immigrating cod from the east, there are currently large spatial differences in cod abundance and mean weight in the western Baltic management unit that raise new management concerns. First, the high abundance of cod of eastern origin found in the western Baltic management unit can mask the relatively poor state of the western Baltic cod population. Second, the current fishing mortality estimates for the entire western Baltic management unit, used as basis for fisheries management, are difficult to interpret as these are highly influenced by mixing of biological populations and the spatial distribution of fisheries. [ABSTRACT FROM PUBLISHER]

Twentieth century mark-recapture studies of conventionally tagged plaice suggested that the maximum exchange rate of fish between the North Sea and the eastern English Channel did not exceed 10%. However, recent studies with electronic data storage tags have suggested that this figure may be significantly greater. In spite of this, current stock assessment and management advice is predicated upon the assumption that the eastern English Channel and North Sea contain two distinct stocks, with no mixing between them. Previous studies on other stocks have shown that different mixing rates can yield grossly different predictions of abundance trends. Using a simulation framework based on the current management regime and simple assumptions regarding the mixing scenario, the consequences of ignoring migratory behaviour between independently managed stocks was investigated. Results indicate that the effects of stock mixing can generate considerable bias in the perceived state of a stock relative to its true state and that changes in the management applied to one stock may cause changes in the level of bias for neighbouring stocks. However, it is concluded that improved management will not necessarily be achieved by developing increasingly complex spatial and temporal assessment methods but rather by developing simple but robust management strategies that have been evaluated before implementation against plausible hypotheses about the dynamics of the stocks and fisheries to be managed. Evaluation should be undertaken using a simulation framework that allows a range of plausible hypotheses about stock and fishery dynamics to be included. [Copyright &y& Elsevier]

• Otolith chemistry techniques were used to characterize the origin of Atlantic bluefin tuna. • Most fish caught in the Gulf of Maine originated from the east (Mediterranean Sea). • Smaller size fish were eastern origin; however, the largest fish were more likely western origin. • Mixed stock information was used to revise U.S. rod and reel fishery harvest data. The highly migratory Atlantic bluefin tuna (Thunnus thynnus) has a distribution that spans the North Atlantic and two distinct spawning populations, an eastern population originating in the Mediterranean Sea and a western population originating in the Gulf of Mexico. Atlantic bluefin tuna are managed as two separate management units (east and west) in the North Atlantic, despite observed mixing that occurs across the management boundary. Characterizing the effects of stock mixing has been identified as a priority for improving the management of Atlantic bluefin tuna. Identifying the stock composition of landings from the Gulf of Maine is of particular importance, because approximately 70 % of the U.S. western Atlantic total allowable catch is removed from this region annually. The aim of our research was to apply otolith chemistry techniques to characterize the origin of bluefin tuna caught in the U.S. rod and reel fishery in the Gulf of Maine and to demonstrate how this information can be applied in fisheries management. Prior research established otolith stable isotope chemistry (δ 13C and δ 18O) as an effective and reliable stock identification tool, and we applied this approach to determine the population of origin of bluefin tuna collected from fishery dependent sampling (recreational and commercial) in the Gulf of Maine. Results indicated that the majority of fish caught in the Gulf of Maine from 2010 to 2013 were eastern origin. We found the highest proportion of eastern origin fish were caught in 2012 and the proportion of eastern origin fish was greater in late summer to fall. Although the majority of fish in small and intermediate size classes were eastern origin, fish in the largest size class (>250 cm) were predominantly western origin. Using these data, we demonstrated an approach for integrating mixed stock composition information into fishery-specific harvest data (U.S. rod and reel catch, catch-at-age, and catch-per-unit-effort). This information can be used to monitor mixed stock composition of the fishery, partition catch to population of origin, and to inform management decisions aimed at controlling population of origin harvest. [ABSTRACT FROM AUTHOR]