Strong Year-classes Research Articles

On features of the life cycle of humpback shrimp Pandalus hypsinotus (Decapoda, Pandalidae) in the Tatar Strait

The study is based on biological analyses of > 80,000 ind. of Pandalus hypsinotus collected in trawl surveys and trap fishery in the Tatar Strait in 2010–2022. Due to the lack of age-recording structures, the size-age structure of shrimp is usually determined by various statistical methods, each of which has some limitations and drawbacks. The modal analysis of size-sex composition dynamics is used in this study, including tracing the growth of strong year-classes. The analysis takes into account the dynamics of major seasonal events such as group molting, spawning, and larval hatching. The reproductive cycle of females is assumed to be 2 years. Based on results of the analysis, the life cycle of humpback shrimp in the Tatar Strait is as follows. After the larvae hatch from eggs in April-May, immature shrimp spend the first year of their life at the depths < 70 m, where they reach the carapace length of 9–15 mm, with a modal size of 11–12 mm. In the second year, they grow to the length of 20–22 mm, on average, then become males and migrate to deeper layer where join the adults. Further growth is described on the data for the strong year-class of 2010: the modal size of males reaches 26–27 mm at age 3 years, 29–30 mm — at age 4, 32–33 mm — at age 5, and 35–36 mm — at age 6. Gender transition of shrimp begins at age 5+, but most males become hermaphroditic at the age 6+. The modal size of intersex hermaphrodites ranges from 37 to 39 mm. At the age of 6 years and 9 months, the intersexes spawn en masse and transform into females. The first larval hatching of new females occurs after 15 months, i.e. at exactly 8 years of age. At least three cycles of spawning (larvae release) are assumed for females, taking into account their polymodal size composition and dynamics of their size increments per molt by age. In assumption of 2-year reproductive cycle, the female stage lasts 6 years. Thus, the total life span for the most of humpback shrimps is estimated to be 12 years (6 years before gender transition and 6 years after). Apparently, a small number of them live 14 years and spawn four times, and some can live up to 16 years and spawn five times.

Thiamine Deficiency M74 Developed in Salmon (Salmo salar) Stocks in Two Baltic Sea Areas after the Hatching of Large Year-Classes of Two Clupeid Species—Detected by Fatty Acid Signature Analysis

Lipid-related thiamine (vitamin B1) deficiency of Baltic salmon (Salmo salar), the M74 syndrome, is generally caused by feeding on abundant young sprat (Sprattus sprattus) in the Baltic Proper, the main foraging area of these salmon. In 2014, a strong year-class of sprat was hatched in the Baltic Proper, and a strong herring (Clupea harengus) year-class was hatched in the Gulf of Bothnia, where herring is the dominant salmon prey. The fatty acid (FA) signatures of prey fish in muscle or eggs of second sea-year spawners suggested that 27% of wild River Simojoki and 68% of reared River Dal salmon remained in the Gulf of Bothnia in 2014 instead of continuing to the Baltic Proper. In 2016, 23% of the M74 females of the River Simojoki and 58% of the River Dal originated from the Gulf of Bothnia, and 13% and 16%, respectively, originated from the Baltic Proper. Some salmon from the River Neris in the southern Baltic Proper had also been feeding in the Gulf of Bothnia. In general, low free thiamine (THIAM) concentration in eggs was associated with high lipid content and high docosahexaenoic acid (DHA, 22:6n−3) and n−3 polyunsaturated FA (n−3 PUFA) concentrations in muscle but not in eggs. A higher THIAM concentration and lower proportions of DHA and n−3 PUFAs in Arctic Ocean salmon eggs, despite higher egg lipid content, indicated that their diet contained fewer fatty fish than the Baltic salmon diet. Hence, M74 originated by foraging heavily on young fatty sprat in the Baltic Proper or herring in the Gulf of Bothnia.

Commercial resources of saffron cod <i>Eleginus gracilis</i> in the Russian waters of the Far-Eastern Seas

Saffron cod Eleginus gracilis, being a typical representative of the elittoral ichthyocene, is an important commercial species in the coastal areas of the Far-Eastern Seas. Small- and medium-sized vessels equipped with both active fishing gears as Danish seine or trawl and fixed nets are used for fishery of this species, it is also popular for amateur and sport fishing from the sea ice. The portion of saffron cod in the total landing is the largest in the Karaginsky fishing subzone (8.1 %) and West-Kamchatka fishing subzone (3.6 %). Annual catch of the species in the Far-Eastern Seas fluctuated in 2003–2022 from 16.7 . 103 to 50.6 . 103 t, on average 33.9 . 103 t. The bulk of this value was caught in the waters at Kamchatka, mostly at its western coast where the average catch was 10.5 . 103 t annually in the last decade. The main fishing grounds of saffron cod are located within the Kamchatka-Kuril, West-Kamchatka, East-Sakhalin and Karaginsky fishing subzones, whereas the stocks at Primorye and sparsely populated coasts of the western Okhotsk Sea and northwestern Bering Sea are underexploited. The saffron cod CPUE reaches 4.5 t per hour of trawling on the shelf of West Kamchatka. The biomass of saffron cod is estimated recently as 197.4 . 103 t in the West-Kamchatka subzone, 100.4 . 103 t in the Karaginsky subzone, 32.8 . 103 t in the West Bering Sea fishing zone, 17.2 . 103 t in the Petropavlovsk-Commander subzone, 17.1 . 103 t in the East-Sakhalin sunzone, and 8.7 . 103 t in the North Okhotsk Sea subzone. The species is distinguished by significant fluctuations of commercial stocks caused by both environmental impacts on its recruitment and fishing pressure. The strong year-classes appear usually in relatively warm or moderate environmental conditions. In modern times, abundance and biomass of almost all exploited stocks of saffron cod in the Far-Eastern Seas excess their average long-term level or fluctuate around, so the state of their populations is considered as quite satisfactory.

Temperature Effects on Recruitment and Individual Growth of Two Antagonistic Fish Species, Perch Perca fluviatilis and Roach Rutilus rutilus, from a Climate Change Perspective

Perch (Perca fluviatilis) in four boreal lakes and sympatric roach (Rutilis rutilis) in two of these lakes were studied by means of gill net sampling, and age and growth were analyzed to explore whether summer temperature affects recruitment and individual growth of the two species differently. Water temperature logging in the lakes showed a highly significant correlation with air temperature. Year-class strength of the perch was significantly positively affected by the summer air temperature, whereas the temperature effect was non-significant for the roach. There was a significant negative effect of year-class strength of one-year-old perch on roach year-class strength in one lake. ln one of the allopatric perch populations, the year-class strength correlation with temperature failed after biomass removal, probably because fishing affected the age structure. July–August air temperature had a significantly positive effect on individual growth rates of two- to five-year-old perch, whereas the effect on roach growth was less clear. The inclination of perch to predate its competitor, roach, as well as conspecifics, as well as accelerated individual growth, were shown to occur more frequently in years with abundant 1+ perch, i.e., the summer after the rise of a strong year-class. Climate warming may potentially favor perch recruitment and growth.

On state of commercial herd of chum salmon <i>Oncorhynchus keta</i> and forecasting its abundance at Iturup Island

Population dynamics and size-age structure are analyzed for the chum salmon herd formed in the Prostor and Kurilsky Bays of Iturup Island (Kuril Islands). Release of chum juveniles into the waters of the bays increased from 10.7.106 ind. in 1996 to 197.6.106 ind. in 2020. The catch of adults was closely related with the release of juveniles (r = 0.66) and increased from 800 t in 1996 to 18,968 t in 2019. Year-to-year dynamics of the chum salmon catch coincides in these two bays, i.e. strong year-classes provide good catch in both bays and low survival causes low catch everywhere. The spawners of age groups 2+ and 3+, mostly 3+ (62 %, on average) dominated in the catches in 2014–2022. This feature makes available the forecast of chum salmon landing in the Prostor and Kurilsky Bays because of strong dependence of annual catch of fish at the age 3+ on the catch at the age 2+ in preceding year (r = 0.89; rs = 0.98). Besides, a significant relationship is noted between the annual catches of chum and pink salmon belonged to the same year-class, i.e. between the catches of chum salmon at the age 3+ and the catches of pink salmon in preceding year (r = 0,71; rs = 0,75).

Modern status, formation and exploitation of walleye pollock stocks off the southern Kuril Islands

The aim of the study is analyze the state and clarify modern status of walleye pollock stocks in the southern Kuril Islands waters.Result: Strong year-classes are revealed, which determined pollock stock dynamics in the South Kuril region in the 2010s — e arly 2020s. It is shown, that the maximum pollock biomass since the early 1990s was observed in the early 2010s, and in the early 2020s decreased to the average level. The organization of fishing and the composition of pollock commercial catches off the southern Kuril Islands are differ at low and high levels of stocks.Novelty: New data on structure and pollock stocks assessment in the South Kuril region since 2011 based on the integrated surveys results are presented. Long-term data on the effectiveness of fishing and the structure of pollock commercial catches in the South Kuril zone are considered.Practical significance: The obtained estimates of the current state and structure of pollock stock will be used for short-term and long-term forecasts.Research methods: walleye pollock stocks assessment in the South Kuril region is based on the annual integrated research expeditions in accordance with the standard methods used in TINRO to monitor pollock stocks.

Oceanographic conditions within the main area of walleye pollock

Purpose: to characterize oceanographic conditions and their variability at various time scales within the walleye pollock area, including all Far Eastern seas of Russia, the Chukchi Sea, and the South Kuril and East Kamchatka regions.Methods: analysis of literary sources and data of expedition studies conducted by the Far Eastern branches of «VNIRO».Novelty: for the first time, a generalization of current information on the oceanological conditions of walleye pollock habitat in the main areas of its reproduction, feeding and fishing was given.Results: In the Bering Sea, three periods were identified during 1950–2020: cold period (1950–1976), moderate period (1977–2013) and warm period (2014–2020). Strong year-classes of walleye pollock were formed only over the moderate period. During the modern warming of the Bering Sea migrations of walleye pollock from the US EEZ into Russian waters occurs by one month earlier that favors the pollock fishery in the Navarin area and Gulf of Anadyr. The warming of Okhotsk Sea waters, combined with the tendency to reduction of its ice cover, favor expansion of the walleye pollock feeding area northward and westward. The warming of the Sea of Japan results in a depressive trend for all Japan Sea walleye pollock populations. At the same time, against the background of depression in modern (abnormally warm) conditions, the formation of single strong year-classes of any Japan Sea walleye pollock population is still possible.Practical significance: the results of this work can be used to improve the methods of the medium-r ange and long-range forecasts of walleye pollock catch.

Reproduction, stock status and fishery for walleye pollock in Peter the Great Bay in the modern period

The aim of the study is to analyze the current state of reproduction, resources and pollock fishery in Peter the Great Bay.Result: It is shown that the increase in pollock abundance is associated with the appearance of a strong yearclass of 2014 in Peter the Great Bay. Pollock biomass has increased significantly, mass spawning has resumed in traditional spawning areas, commercial catch of pollock increased to record levels. However, during the three spawning seasons of the year-class 2014 there was no significant recruitment, so the fish stock is expected to return to a low level.Novelty: The results of scientific research conducted in the Peter the Great Bay in 2019–2022 made it possible to obtain new data on the reproduction, current state and structure of pollock stock.Practical significance: The obtained estimates of the current state and structure of pollock stock will be used for short-term and long-term forecasts.Research methods: The data of research bottom trawl and ichthyoplankton surveys and pollock catch statistics from the Industry Monitoring System (OSM «Fishing») were used.

Bomb Radiocarbon, Otolith Daily Increments and Length Modes Validate Age Interpretations of Chilean Jack Mackerel (Trachurus murphyi)

Ages of Chilean jack mackerel (Trachurus murphyi) in the southern Pacific Ocean were validated using three methods: 1) daily microincrement readings in sagittal otoliths of young-of-the year (YOY) fish to validate the first annulus; 2) modal progression of strong year-classes (PSYC) to validate the first, second and third annuli, and 3) bomb radiocarbon analysis of otolith cores to validate the absolute age in older fish >38 cm fork length (FL). A Laird-Gompertz (LG) model was fitted to fork length (cm) at age (days) relationship in fish ranging from 3.4 to 25.0 cm FL and from 56 to 550 days in age. The LG model estimated a mean FL of 22 cm at the end of the first year of life, which was large compared with the mean FL estimated through conventional ageing in whole otoliths. The comparison between daily age with annual growth bands from whole otoliths, showed a false increment formed at 185 ± 34 days and a second translucent increment formed at 352 ± 79 days, corresponding to the first annulus for fish with the mean FL of 21.4 ± 1.8 cm. The PSYC coinciding with the recruits of 2008 (age 0), was tracked through three subsequent years with high accuracy, attaining 23, 27 and 30 cm FL in 2009, 2010 and 2011, respectively. These modes identified as age 1, age 2 and age 3 in the PSYC, matched to the mean length of fish with 2, 4 and 5 translucent increments in sagittal otoliths. Results from bomb radiocarbon analysis confirmed that most of the ages were correct, because the Δ14C content of adult otolith core matched the reference chronology. The absolute mean ages for the assayed otoliths, based on the comparison with the reference chronology, ranged from 7 to 13 years for sizes between 39 and 60 cm FL. The age validation for T. murphy as addressed in the current study provided crucial information for enhancing stock assessment output of this important transboundary commercial species.

Long-term dynamics of forage base and food supply for nekton in the upper epipelagic layer of the western Bering Sea. Part 2. Food supply for nekton

Interannual dynamics of feeding conditions for nekton in the upper epipelagic layer of the western Bering Sea is considered for the summer and fall seasons of 2002–2020. In the summers, daily consumption by fish and squids ranged from 0.01 to 0.10 t/km2 in the deepwater basins and between 0.01–0.38 t/km2 in the shelf area at Cape Navarin. In the falls, the consumption increased to 0.03–0.14 t/km2 in the deep-water basins, and to 0.01–0.53 t/km2 in the Navarin area. Summary in the summer-fall periods, the fish and squids consumed 2–5 % of zooplankton production (up to 5–7 % in some years) in the deep-sea basins that is rather low degree of the zooplankton resources usage. In the Navarin area, the consumption is considerably higher and is estimated as 5 % in summer and 21 % in fall season, on average. In cases of heightened nekton biomass and reduced zooplankton biomass in some years, the grazing pressure on plankton increased, in particular on euphausiids, hyperiids and pteropods, while the intensity of salmon feeding decreased, with extension the portions of nektonic food and other zooplankton groups, as arrowworms, in their diet. On the other hand, the portion of juvenile fish and squids in the salmon diet increased in cases of high abundance of this prey, too. Weak correlation was detected both between the salmon feeding and abundance of their prey and between the salmon feeding and abundance of salmons. The feeding conditions for pink salmon during their offshore migration did not determine the rate of their returns. Strong year-classes of pink salmon had large returns in conditions of either poor or rich food supply in fall season, so the feeding in fall was not important for the pink salmon survival in the ocean in winter. Feeding of other fish and squids species did not change noticeably in the years with reduced food supply. In general, abundance and species composition of plankton and nekton communities were rather stable in the western Bering Sea in the 2000-2010s, and this current state of the ecosystem provides stable functioning of nekton communities both in the deepwater and Navarin areas.

Two-Year Cycle of Vendace (Coregonus albula) in Pyhäjärvi, SW Finland: Evidence for Asymmetric Competition Between Adults and Juveniles

Long-term monitoring of the planktivorous vendace (Coregonus albula) in Pyhäjärvi, a lake in SW Finland, revealed periods characterised by different types of population regulation. In 1971–1989, the vendace stock was strong and exhibited a two-year cycle. In 1990–1999, extreme weather conditions and predation resulted in recruitment failures, after which overfishing kept year-classes small. From 2000 onwards, the two-year oscillations were re-established at a lower level. Here, we show that the two-year cyclicity prevalent in the 1980s was consistent with the hypothesis of asymmetric competition between adults and juveniles. Food consumption by juveniles of strong year-classes retarded growth and weakened condition of the co-occurring adults, resulting in less abundant year-classes. In 2000–2018, the role of intraspecific competition diminished due to interspecific competition from increased populations of other planktivorous fish, preventing vendace from attaining higher abundance. Elevated temperature probably confounded the effects of competition, but its direct role was masked by simultaneous gradual eutrophication and fish assemblage changes.

Long-term dynamics for stocks of sablefish Anoplopoma fimbria in the western Bering Sea and prospects for their fishery

Sablefish is an endemic species of the North Pacific. Its range extends from California Peninsula, along the Pacific coast of the US and Canada to Aleutian Islands and further, along the Pacific coast of Kamchatka and the Kuriles to the central part of Honshu Island. They dwell also in the Bering Sea and southeastern Okhotsk Sea. Sablefish are the most abundant in the southeastern Bering Sea and in the Gulf of Alaska, that is conditioned by favorable conditions for their larvae and juveniles. In the Asian part of the range, the environments are generally more severe, and reproduction of sablefish is rather risky. Following to the results of modern genetic studies, the sablefish stocks are distinguished by high genetic homogeneity that suggests a common population with the main spawning grounds in the southeastern Bering Sea, at the Pacific coasts of Aleutian Islands, in the Gulf of Alaska, and at the coasts of British Columbia, Washington, Oregon and California. Dynamics of the sablefish biomass is considered on the data of bottom and midwater trawl surveys conducted by TINRO in 2003–2020, fishery statistics, and accessible data of NOAA (USA). Sharp increasing of the biomass and annual catches is noted both in the eastern and western Bering Sea in the last few years because of appearance of several strong year-classes. Western Bering Sea stock depends on migration of recruits from the common spawning grounds in the southeastern Bering Sea. For the western Bering Sea, two main ways of such migration are possible: i) active migration of juveniles with benthic habitat; and ii) passive transfer of pelagic larvae and early juveniles across the Bering Sea through the system of surface currents. The latter mechanism supports the sablefish recruitment in the bays of the western Bering Sea and, to a lesser extent, at the eastern coast of Kamchatka. Sablefish in the West Bering Sea fishery zone were caught in 2010–2020 mostly as by-catch for trawling and longline fishery (93 %), other 7 % were landed by specialized longline fishery. The basic points for managing the sablefish fishery in the West Bering Sea zone are defined. About 400 t of sablefish is permissible to catch annually in the West Bering Sea fishery zone in conditions of modern high stock of this species. This value includes 100–120 t that will inevitably be caught as by-catch and the rest of 280–300 t is a foreseeable resource for organization of specialized fishery.

The Status of the Stock and the Current Fishery for the Atka Mackerel, Pleurogrammus monopterygius (Pallas, 1810), in the Olyutorsky–Navarin Area, Bering Sea

The dynamics of the Atka mackerel stock in the Olyutorsky–Navarin area in 1994–2019 is inferred from bottom-trawl surveys, fishery statistics, open-access NOAA data, and the results of simulation by the method of virtual population analysis. After the period of low stock in the mid-1990s, the abundance of the Atka mackerel group in the area increased sharply to a maximum in 2006–2008, when the spawning biomass was estimated at approximately 9500 t and the fishery biomass at 14 000 t. In 2008, the stock showed a tendency to decrease; by 2013, the spawning stock decreased to 3600 t and the fishery stock to 5600 t; subsequently, there was a period of stabilization at a low level with a slight tendency to further reduction. One of the possible causes of the sharp increase in the Atka mackerel stock in the 2000s could be the increased transport of juveniles of strong year-classes from the main spawning grounds off the Aleutian Islands with their subsequent settlement on available parts of the shelf off Cape Olyutorsky. The development of the Atka mackerel fishery in the Olyutorsky–Navarin area in 1994–2019 had a pattern similar in general to the stock dynamics. Until 1995, no more than an average of 100 t were caught per year; in 2006–2010, the average annual catch increased to 790 t; in 2011–2015, it increased to 1150 t; and in 2016–2019, the catch again decreased to 950 t. Due to the emerging trend of a general reduction in the stock of the Aleutian population, this low level of catches in the Olyutorsky–Navarin area is expected to continue in the coming years.

The Validity of Otolith-Based Age Estimates for the Walleye Pollock Theragra chalcogramma (Pallas, 1814) from the Eastern Sea of Okhotsk

Data on age of the walleye pollock Theragra chalcogramma (Pallas, 1814) from the eastern Sea of Okhotsk, determined by otolith analysis, are considered. A comparison of the age–length relationships, as inferred from variations in modal body size of fish of strong year-classes and from otolith-based age estimates, has shown no significant differences. This confirms the accuracy of the otolith-based estimates of walleye pollock age.

SIZE COMPOSITION AND GROWTH RATES OF BLUE KING CRAB PARALITHODES PLATYPUS BRANDT, 1850 IN THE RUSSIAN SECTOR OF THE BERING SEA

Size composition and growth rate of blue king crab males are analyzed on the data of 8 bottom trawl surveys conducted in the western Bering Sea in summer-fall seasons within the period from 2005 to 2017. Significant changes in size structure of this population are revealed. Portion of commercial males was the lowest (18.7 %) in 2010 because of the strong yearclass appeared in the catches in 2005, when its males carapace had the width 20–40 mm, that provided in 2008–2010 the percentage of juveniles with the size < 100 mm about 59–60 %. Then this strong year-class reached the commercial size, so the portion of commercial males increased to 66 % in 2014 and to 85 % in 2015–2016. The portion of juveniles was extremely small (3.1–6.5 %) in these years. The portion of commercial males had reduced again to 67 % in 2017 because of a new strong year-class appearance that provided the juveniles percentage increasing to 24.6 %. The mean size of commercial males has increased since 2012 in 18 mm and reached the value of 160.4 mm in 2017. Age of the crab males entry into the commercial cohort is estimated as 8 years. The observed cyclic changes in size composition of blue king crab and estimated value of its growth rate indicate indirectly a good and stable state of its population in the western Bering Sea.

Specific Features of the Formation of the Year-Class Abundance of Northeastern Arctic Haddock Melanogrammus aeglefinus during the Warming of the Barents Sea

Factors that influence the survival rate of northeastern Arctic haddock Melanogrammus aeglefinus at different stages of the formation of the year-class abundance are considered. It is shown that the patterns of the formation of the year-class abundance of haddock have changed due to warming in the Barents Sea. Since the late 1990s, an enhancement of the relationship is recorded between the year-class numbers at the 0-group stage and at age 3 years. The mortality of this fry during the first two wintering periods has lost its dominant value for the emergence of strong year-classes. Under these conditions, the predation role of cod Gadus morhua as a regulator of the abundance of haddock demersal juveniles increased. This is due to the fact that the increase in water temperature in the Barents Sea provides favorable conditions for the abundance growth not only for haddock but also for its main predator, i.e., cod.

Is Barotrauma an Important Factor in the Discard Mortality of Yellow Perch?

Abstract In physoclistous fishes, barotrauma caused by rapid decompression during capture may be an important source of fishing mortality that is unquantified for some fisheries. We developed a predictive logistic model for barotrauma incidence in Yellow Perch Perca flavescens and applied this model to Ohio's recreational and commercial fisheries in Lake Erie where fisheries managers implicitly consider discard mortality to be negligible in current stock assessment. As expected, capture depth explained most of the variation in incidence, with comparatively small effects of season, sex, and size categories. Measurements of whole body and gonad density provided limited explanation for the categorical effects. Both fisheries spanned a range of depths (7.6 to 16.8 m) that corresponded to a broad range of barotrauma incidence (13 to 74%). Using a recent example, we estimated that additional fishing mortality due to barotrauma in discards was approximately six-fold higher in the commercial than recreational fishery. Overall, this additional mortality was <1% of lake-wide population size estimates. Thus, the assumption that all discarded Yellow Perch survive is unlikely to result in a detectable bias in population estimates. One caveat is that we still do not understand how strong year-classes might influence discard mortality via increased discard rate and barotrauma incidence for small fish.

The dynamics of the abundance of the walleye pollock Theragra chalcogramma (Pallas, 1814) (gadiformes: gadidae) in the Sea of Japan

The biomass of the walleye pollock (Theragra chalcogramma) stock in waters of Primorskii krai, Sea of Japan, during the 1976–2015 observation period ranged from 48000 to 373000 tons; their number ranged from 99 to 1115 million fish. Four very strong year-classes born in 1975, 1981, 1997, and 2006 have been identified. It has been shown that the duration of the sexual-maturity period is determined by the periodicity of the dynamics of the year-class strength close to the 9-year cycle. According to the results of numerical modeling, an increase in the walleye pollock stock in Primorye is expected from 2017 to 2020.

Dynamic population trends observed in the deep-living Pacific flatnose, Antimora microlepis, on the U.S. West Coast

As fisheries managers attempt to incorporate ecosystem-based considerations into decision making, it is important to understand the role that non-target species play in the ecosystems that support commercial fisheries. For some deep-water groundfishes, basic information on biology and population dynamics is extremely limited. This study presents findings on the spatial distribution, growth trends, and relative abundance of the Pacific flatnose, Antimora microlepis, using data collected from 2003 to 2015 by the Northwest Fisheries Science Center's West Coast Groundfish Bottom Trawl Survey (WCGBTS). We observed a 67% increase in mean fork-length over the study period reflecting the advancement of strong year-classes from the early 2000s that currently dominate the population as a whole. Catch-weighted depth increased significantly as these cohorts migrated to deeper waters of the continental slope. Although catch per unit effort remained relatively constant, this demographic shift suggests that episodic recruitment may affect the resilience of this stock to fishing mortality over time. A notable decrease in the percentage of females observed after 2012 seemed to indicate the movement of large, older females to depths beyond the 1280m limit of the survey. Otolith weight provided a useful proxy for age in growth models for this species.

Age-related otolith chemistry profiles help resolve demographics and meta-population structure of a widely-dispersed, coastal fishery species

Snapper, Chrysophrys auratus, is a highly important fishery resource of southern Australia. Through the late 2000s, the State of South Australia (SA) produced record commercial catches of this species, making the dominant contribution to the national catch. This was associated with dramatic changes in contributions from SA’s regional fisheries. The demographic processes responsible for these unprecedented changes were not understood, reflecting poor understanding of large-scale movement of snapper and its influence on regional population dynamics and stock structure. This study used otolith chemistry to investigate the movement behaviour and stock structure of snapper in the eastern part of southern Australia. Transverse sections of otoliths from four year classes and six geographic regions distributed over 1500km of coastline were sampled by continuous life-history profiles across the chronological structure using laser ablation inductively coupled plasma mass spectrometry. Element:calcium ratios were integrated with annual increment measurements to produce profiles of age-related, annual averages for Ba:Ca, Sr:Ca, Mn:Ca and Mg:Ca. Significant regional differences for each annual zone were used to infer differences in fish origins and inter-regional movement. Variation in Ba:Ca was most informative and indicated that an episodic fishery along the south east coastal region of SA is linked to periods of stock range expansion, involving movement over 600km, from a major nursery area, Port Phillip Bay (PPB), in the adjacent state of Victoria. Periods of stock range expansion coincided with exceptionally strong year-classes originating from spawning in PPB. Inferences from the element:calcium profiles and other information suggested that the populations in the northern parts of the two gulfs of SA appear to be separate, self-recruiting and independent of inter-regional movement. Each, however, is an important nursery area and source of emigration that replenishes regional populations in adjacent coastal waters. The results support SA’s snapper fishery being divided into three stocks, each dependent on a primary nursery area. The source for the intermittent fishery in the south east region of SA is within a separate management jurisdiction (Victoria), necessitating consideration of multi-jurisdictional assessment and management. We hypothesise that density-related processes are responsible for range expansion of this multi-jurisdictional stock, with strong year classes producing more extensive emigration and range expansion into SA waters. The study provides an example of the value of life-history otolith chemistry profiles for resolving stock structure at regional scales with implications for fisheries assessment and management.