Environmental control of Northeast Atlantic mackerel (Scomber scombrus) recruitment in the southern Bay of Biscay: case study of failure in the year 2000

We investigate the effect of strong meteorological perturbations in early spring on the success of mackerel (Scomber scombrus) recruitment in the N/NW Iberian area (southern Bay of Biscay) for the period 1999–2008. In 2000, the year of the most pronounced recruitment failure on record, two consecutive multidisciplinary surveys sampled hydrographic conditions and mackerel eggs, larvae and post‐larvae over the main mackerel spawning grounds of the north and northwest coast of the Iberian Peninsula. Analysis of egg and larval abundance and birthdates based on the otoliths of mackerel juveniles caught between July and October 2000 showed that there were no survivors from the early spring spawns, indicating a massive loss of early spawning effort. Moreover, the abundance of 1‐year‐old mackerel estimated from an acoustic survey carried out in 2001 was the lowest observed within the 1999–2008 time series. This low or null survival from the early spawns in 2000 could be due to the meteorological and oceanographic conditions of that spring, in particular two storm events in April after a relatively calm March. The first storm event from the north caused strong local wind in the southern Bay of Biscay but a weak oceanographic response. The second storm event from the southwest was mainly felt west of Galicia and caused a notable increase in shelf currents and a shift of the hydrographical structure along the shelf. Detailed analysis of strong wind pulses in early spring within the historical recruitment record suggests that strong local turbulence generated by high wind speeds and advection of larvae caused by the enhancement of shelf currents can contribute to reduced recruitment. Our observations indicate that, in 2000, both mechanisms were present.     

The recruitment process of the Barents Sea capelin (Mallotus villosus) stock, 2001–2003

Oceanographic and predation processes are important modulators of fish larvae survival and mortality. This study addresses the hypothesis that immature Norwegian spring‐spawning herring (Clupea harengus), when abundant in the Barents Sea, determine the capelin reproduction success through consumption of Barents Sea capelin (Mallotus villosus) larvae. Combining a hydrodynamic model and particle‐tracking individual‐based model, a realistic spatio‐temporal overlap between capelin larvae and predatory immature herring was modelled for the summer seasons of 2001–2003. Capelin larvae originating from western spawning grounds became widely dispersed during the summer season, whereas those originating from eastern spawning grounds experienced a rapid drift into the southeastern Barents Sea. Herring caused a 3% mortality of the capelin larvae population in 2001 and a 16% mortality in 2003, but the effect of predation from herring on capelin larvae was negligible in 2002. Despite a strong capelin larvae cohort and a virtual absence of predatory herring, the recruitment from the capelin 2002 year class was relatively poor from a long‐term perspective. We show that the choice of capelin spawning grounds has a major impact on the subsequent capelin larvae drift patterns, constituting an important modulator of the capelin larvae survival. Variation in drift patterns during the summer season is likely to expose the capelin larvae to a wide range of hazards, including predation from young cod, sandeel and other predators. Such alternative predators might thus have contributed to the poor capelin recruitment during 2001–2003, leading to the collapse of the capelin stock in the subsequent years.     

Residency of adult Atlantic cod (Gadus morhua) in the western Gulf of Maine

A mark and recapture study of cod in the western Gulf of Maine was conducted to study the seasonal movements of fish, particularly as they related to areas closed to commercial fishing. A total of 27,772 cod were tagged, and 1334 (4.8%) were recaptured with sufficiently detailed recapture location and date to be included in the study. Results indicated that the group is resident to the area and sedentary. Although there were a small percentage of fish (2.5%) that traveled long distances (>100 km), most were recaptured near their release location. There was no linear relationship between fish length and linear distances traveled, and linear distance traveled was only weakly related to days-at-large. For all groups of cod tagged and released in particular areas and months, mean distances traveled were small (<65 km), rates of travel were slow (<0.2 km/day), and rates of group dispersion were <50 km²/day. There were no recognizable spatial or temporal patterns in the mean angles of travel for groups released in various month/area combinations, and the angular deviations associated with the mean angles were quite large. Although there seems to be little pattern in the movement of cod in our study area, temporal and spatial changes in abundance indicate that movements are occurring.

The general pattern was a concentration of large cod into one small, inshore area (Area 133) in both the spring and winter, and dispersion from this area in the ensuing months. Monthly percentages of ripe cod in Area 133 provide evidence for two spawning groups; a winter group that spawns from November through January, and a spring group that spawns from April through July. Thus it is likely that the observed spring and winter concentrations of fish in Area 133 were associated with spawning. Fish in the spring group were the largest encountered in the study. The study also provided some evidence of natal homing, i.e. a return to the same spawning grounds year after year, for the spring spawning group. Although there is some exchange of fish between most of the closed areas, the timing of the closures appears to protect the largest aggregations of cod.     

Seasonal spawning and wind‐regulated retention‐dispersal of early life stage Atlantic cod (Gadus morhua) in a Newfoundland fjord

Sequential ichthyoplankton surveys were used to determine the spatial and temporal distribution of eggs and larvae over coastal spawning grounds of Atlantic cod (Gadus morhua) in Smith Sound, Trinity Bay, Newfoundland, during the spring and summer of 2006 and 2007. Egg densities showed similar patterns for both years with two peaks in abundance in spring (March–April) and late summer (late July). A clear progression of development stages (1–4) was observed in spring and summer in 2006 and summer in 2007, suggesting retention of eggs within the Sound during these periods. Modelled predictions of vertical egg distributions indicated eggs were broadly distributed near the surface during spring (March–April), but were concentrated below the pycnocline (>10 m) within the inner portions of the Sound during the summer months (July–August). Back‐calculated peaks in spawning based on water temperatures were estimated at 11 and 4 April for 2006 and 2007, respectfully, with late season peaks centred on 21–24 July for both years. Environmental data indicated cooler water temperatures and periods of high wind stress in spring, and warmer, calmer periods late summer, consistent with higher retention and faster development times on the spawning grounds later in the season. We conclude that spring and summer spawning events result in different distributions of early life stages and may lead to different distributions of juvenile and adult fish.     

Dispersal of black sea bass (Centropristis striata) larvae on the southeast U.S. continental shelf: results of a coupled vertical larval behavior – 3D circulation model

In the marine environment, pelagic dispersal is important for determining the distribution and abundance of populations, as well as providing connections among populations. Estimates of larval dispersal from spawning grounds are important to determining temporal and spatial patterns in recruitment that may have significant influences on the dynamics of the population. We present a case study of the dispersal of Centropristis striata (black sea bass) larvae on the southeast U.S. continental shelf. We use a coupled larval behavior – 3D circulation model to compare the effects of the timing and location of spawning against that of larval vertical migration on larval dispersal. Using the results of field data on larval vertical distributions, we compare the dispersal of virtual 'larvae' which have ontogenetic changes in vertical behavior with that of particles fixed near the surface and near the bottom. Larvae were released at potential spawning sites four times throughout the spawning season (February through May) for 3 yr (2002–04) and tracked for the assumed larval duration (from 27 to 37 days including the egg stage). Results indicate that adult behavior, in the form of spawning time and location, may be more important than larval vertical behavior in determining larval dispersal on the inner- and mid- continental shelves of this region.     

Over-wintering growth and losses in a small population of the threespine stickleback, Gasterosteus aculeatus (L.), in mid-Wales

Abstract –  Mark-recapture was used to estimate the population abundance of threespine stickleback, Gasterosteus aculeatus , in a backwater of Afon Rheidol in mid-Wales in autumn and early spring, from 1984 to 1998. For the 0+ cohort, there was no evidence of density-dependent losses (mortality and emigration) or growth in the over-wintering period. No systematic size-dependent loss was detected. There was an inverse relationship between autumn abundance and the annual rate of change in abundance of the 0+ cohort.     

Observation on the spawning of Sardina and Sardinella off the south Moroccan Atlantic coast (21–26°N)

This paper provides information on the early life stages of the Moroccan Atlantic sardine, Sardina pilchardus, and two species of Sardinella: Sardinella aurita and S. maderensis, between Cape Blanc (21°N) and Cape Boujdor (26°N), dealing with the spawning grounds, the nursery areas and the optimal spawning temperature and salinity range.

The spawning areas for Sardina are located North of Dakhla (24°30′–25°30′N) and near Cintra Bay (23°N). The larval development area is described together with seasonal and annual characteristics. The nurseries of Sardina are near Dakhla and to the south between 21 and 22°N but precise locations vary with season. The maximum occurrence of eggs was during winter, and was at temperatures from 16 to 18 °C in winter and between 18 and 18.5 °C in summer.

The main spawning area of Sardinella species is between Cape Blanc and Cintra Bay (21–23°N) with a maximum occurrence of eggs and larvae in July. For Sardinella off the southern region of Morocco, the optimal temperature interval for spawning is between 18 and 21.14 °C. No conclusions can be drawn on the relationship between spawning and water salinity.

The area north of Cintra Bay has a broad, but shallow continental shelf. This topography leads to decreasing dispersion effects and the zone constitutes a favorable area for larval retention and development.     

The effects of survey design and circulation pattern on the perceived abundance of herring larvae: a case study for Norwegian spring spawning herring (Clupea harengus)

A larval survey is used in the annual assessment as an index of the spawning stock size of Norwegian spring spawning herring (Clupea harengus). To test how inter‐annual fluctuations in circulation pattern, survey design and execution of the survey affected the larval abundance estimate we conducted simulated surveys using a model framework with idealized assumptions to model larval drift and sampled larvae using several realistic survey scenarios. The results suggest that inter‐annual variations in circulation pattern alone can have a profound effect on the perception of larvae abundance and that the direction of the survey (north to south versus south to north) can have a significant effect on the estimated abundance, particularly if hatching occurs over a short period of time. Additionally, disruptions to a continuous survey schedule also have an effect and, as such, sampling strategies in case of disruption to the survey are proposed.     

Reproduction and recruitment of the brown shrimp Crangon crangon in the inner German Bight (North Sea): An interannual study and critical reappraisal

The brown shrimp, Crangon crangon, is the most important target of the coastal crustacean fisheries in the German Bight. In order to evaluate the relation between the abundance of ovigerous females and larvae in spring and the recruitment success in autumn, we first analysed the seasonal appearance of ovigerous females and larvae from weekly samples throughout 2012. The spawning season in the German Bight extends over several months comprising multiple unsynchronized spawning events. The minimum shares of ovigerous females appeared in early autumn, and the highest shares in late winter bearing mostly early egg stages. We defined the putative start of the reproductive cycle for November when the frequency of ovigerous females started to increase. There was no distinct separation between winter and summer eggs, but a continuous transition between large eggs spawned in winter (the early spawning season) and batches of smaller eggs in spring and summer. Larval densities peaked in April/May. Consequently, regular annual larval surveys from 2013 to 2016 were scheduled for April/May and extended to six transects covering the inner German Bight. Ovigerous females were most abundant in shallow waters above the 20‐m isobaths, which also explained regional differences in abundance between the regions off North Frisia and East Frisia. No relation was obvious between the number of larvae in spring and recruited stock in autumn. Due to the short lifespan of C. crangon, the combination of various abiotic factors and predator presence seems to be the principal parameters controlling stock size.     

Seasonal movements of silver kob, Argyrosomus inodorus, (Griffiths and Heemstra) in Namibian waters

A seasonal migration pattern for silver kob, Argyrosomus inodorus (Griffiths & Heemstra) is elucidated based on tag-recapture data. Spawning adults start migrating southwards against the north-westerly surface currents at the beginning of the austral summer from the northern end of their distributional range to their spawning grounds, Sandwich and Meob Bays, at the southern end of their distributional range. It is suggested that after spawning larvae drift north with the current to the nursery areas, the West coast Recreational area (WCR). When juveniles reach the age of approximately 2 years they will gradually move north towards the adult feeding ground, the Skeleton Coast Park (SCP) waters. At the end of the spawning season when the surf-zone water temperature decreases to about 15 °C adult silver kob complete their spawning cycle by returning to the adult feeding ground, probably moving slightly offshore and with the current.     

Impact of paralarvae and juveniles feeding environment on the neon flying squid (Ommastrephes bartramii) winter–spring cohort stock

In this study, we found that there were significant positive correlations between the catch per unit effort (CPUE, a squid abundance index) for the neon flying squid (Ommastrephes bartramii) winter–spring cohort and the satellite‐derived chlorophyll a concentrations in their spawning grounds located at 140–160°E where 21°C < sea surface temperature < 25°C from February to May. The spawning grounds of the winter–spring cohort are located in a quiet stream region, and a particle tracking experiment, based on the velocity field obtained from an ocean data assimilation system, showed that paralarvae and juveniles aged <90 days remained in their spawning grounds and the chlorophyll a concentration in their habitat had a significant positive correlation with the CPUE. A backward particle tracking experiment also showed that the chlorophyll a concentration in the spawning grounds had a significant positive correlation with the autumn–winter mixed layer depth. Based on these results, we hypothesize that the CPUE interannual variability is caused by variations in the feeding environment of the paralarvae and juveniles, which may be linked to autumn–winter mixed layer depth variations.     

Population dynamics of portunid crab <Emphasis Type="Italic">Charybdis bimaculata</Emphasis> in Ise Bay,central Japan

ABSTRACT:   The present study dealt with seasonal and interannual variations in the abundance and biomass, and spatio-temporal distributions of the portunid crab Charybdis bimaculata dominant in Ise Bay, central Japan. The abundance and biomass of the crab decreased in summer when the oxygen-poor water developed in central or inner parts of the bay, and then increased through new recruits from autumn (October–November) to the following spring (March–May) when the oxygen-poor water disappeared. Berried females were collected mainly from spring to autumn. Recruits were collected in any season. Particularly in winter, most recruits were located in the innermost part of the bay. According to the cohort separation based on size frequency distribution in carapace width of the crab specimens, the cohorts that were derived from spawning in spring to summer largely contributed to establishing and maintaining the benthic populations for the following year in the bay, whereas those from other seasons failed to recruit because of serious damage caused by the oxygen-poor water. Most crab individuals one year post hatch contributed to spawning and then died by the winter of the same year.     

Impact of environmental deviations on the larval and year-class abundances in the spring spawning herring (Clupea harengus membras L.) of the Gulf of Riga (Baltic Sea) in 1947–2004

Regular weekly sampling of larvae with a Hensen net in the stationary stations on the main spawning grounds of the brackishwater herring population in the Gulf of Riga was carried out in 1947–2004. Probably due to the rapidly increasing anthropogenic eutrophication and the rising winter temperature the yearly abundance of herring larvae has significantly increased with a shift in the average abundance in 1972/1973. Correlation of the average number of herring larvae with the year-class strength was significant for the intermediate (10–16 mm, P < 0.05) and large (≥17 mm, P < 0.001) larval size groups. The monthly average air temperature in the period from January to March was the main environmental variable regulating the abundance of the year classes (P < 0.001). The dependence between the spawning stock biomass (SSB) and year-class abundance differed by climate periods. In the period of cold winters environmental conditions dominated in determination of the year-class abundance whereas the role of the SSB was nonsignificant (P > 0.05). In the period of milder winters the importance of the SSB in determination of the abundance of year classes considerably increased and it significantly correlated with the abundance of year classes (P < 0.05). The winter–spring temperatures and other climatic variables presumably control the abundance of the forming year-class during the period of the highest larval mortality mainly via the production of their food.     

Annual and seasonal dynamics of fish in the brackish-water Matsalu Bay, Estonia

Abstract –  Matsalu Bay in the Väinameri (West-Estonian Archipelago Sea) is a relatively shallow but large bay. It is the only real delta estuary in the northern Baltic Sea. The fish fauna is strongly dominated by freshwater species such as perch Perca fluviatilis , roach Rutilus rutilus , white bream Blicca bjoerkna and rudd Scardinius erythrophthalmus . The study presents CPUE data describing annual and seasonal variations in the species composition, abundance and spatial distribution, based on 1 year of monthly samplings from August 2001 to August 2002 and routine coastal monitoring (in late July–early August) between 1994 and 2003. The bay can be divided into three rather distinct parts. The fish fauna of the inner part (salinity 0–2 p.p.t.) is different from two open parts and resembles that of freshwater lakes. In the openmost part (salinity usually 4–6 p.p.t.) some marine species, such as herring Clupea harengus membras , flounder Platichthys flesus and eelpout Zoarces viviparus may seasonally be abundant. Species composition and abundance of most species varies seasonally and interannually. Only the abundance of few species such as pike Esox lucius and pikeperch Stizostedion lucioperca was not impacted by seasonality. The most dynamic period in the bay is spring, when several species (some of them very rare in summer, such as herring and smelt Osmerus eperlanus ) enter the bay for spawning.     

Tests of larval retention in a tidally energetic environment reveal the complexity of the spatial structure in herring populations

Dispersion during the larval phase is of central importance in the dynamics of marine fish and invertebrate populations. Rapid transport or dispersion of larvae may contribute to connectivity and mixing, whereas spatial persistence (retention) is hypothesized to favour stock complexity and local subpopulations. Larval retention, while rarely quantified, may be defined in species with protracted spawning by the spatial co‐occurrence of larvae of different sizes or ages. The spatial distributions of larval Atlantic herring (Clupea harengus) were examined from 22 annual autumn surveys (1975–1998) and 9 spring surveys (1975–1984) from the Bay of Fundy, a region with large tides and residual flow. Larvae of all sizes (3–27 mm in length, from hatch to nearly 4 months post‐hatch) were observed each year in two major aggregations; one off southwestern Nova Scotia, and the other in the mid‐inner Bay of Fundy off the northwestern shore of Nova Scotia. Two similar aggregations were evident over 5 months later from 9 spring surveys (1975–1984), despite the residual flow that would have swept the larvae from the region within 1 month. Larval retention was apparent from overlapping centres of mass of different size (=age) classes of larvae, and tested using a size diversity index, based on the co‐occurrence of 1‐mm‐size categories, derived from protracted spawning of several weeks. Geospatial ‘hot spots’ (G_i* statistic) of four size (age) classes were evident at specific stations in the 50–100 m bathymetric zone and not elsewhere. These metrics provide quantitative measures of retention that may be applied to many ichthyoplankton data sets. One of the three main spawning areas collapsed during the study period after a period of intense fishing and failed to rebuild, but there was no substantial change in the location of larval hotspots in subsequent years. While larval retention does not directly relate to each spawning location, larval retention in the Bay of Fundy contributes to the complex ‘metapopulation’ structure of herring stocks in the western Atlantic.     

The influence of life history dynamics and environment on the determination of year class strength in North Sea herring (Clupea harengus L.)

The inter-annual variability in year class strength (1976–2000) of North Sea herring (Clupea harengus) was investigated using Paulik diagrams based on survey data and Virtual Population Analysis. The herring life cycle was split into five stages: spawning stock biomass (SSB), egg production, larvae, fish with 0 winter rings on the otolith (0-wr), 1-wr and 2-wr. Surveys were used as indices and Paulik analysis revealed relationships between stages. In 80% of the years, year class strength reflected SSB. Poorer than expected year classes were determined during the larva to 0-wr phase, whilst stronger than expected year classes were apparently determined during the 0-wr to 1-wr stage. There was no clear relationship between survival of young stages of herring and the abundance of Calanus finmarchicus but the year class strength of 0-wr and 1-wr had a negative relationship to bottom water temperature. Lower sea water temperatures in the North Sea are associated with higher Calanus abundance. The analysis shows that the strength of aberrant year classes of North Sea herring is determined between the pelagic larval and the juvenile stages.     

Influence of environmental variables on the distribution of Squatina guggenheim (Chondrichthyes, Squatinidae) in the Argentine–Uruguayan Common Fishing Zone

The environmental variables affecting the spatial distribution of Squatina guggenheim Marini, 1936, an important commercial resource, are unknown. Here the influence of temperature, salinity and depth on the distribution of S. guggenheim in the Southwest Atlantic was analyzed. The species–habitat associations were evaluated for the S. guggenheim population as a whole as well as segregated by sex and by size classes (group 1, 23–44 cm; group 2, 45–74 cm; group 3, 75–91 cm L_T). Sampling took place during four bottom trawl surveys conducted in the Argentine–Uruguayan Common Fishing Zone (AUCFZ) during spring (October 1997) and fall (March 1997, March–April 1998, May–June 1998). The sharks were captured using an Engel bottom trawl and the hydrographical information was obtained using a CTD. Bottom depth was measured with an echo-sounder. The distribution of the population was affected (P ≤ 0.05) by temperature during spring and fall, while it was affected by salinity only in spring. S. guggenheim showed preference for temperatures ranging from 7.0 to 18.5 °C and salinity values ranging from 33.4 to 33.8. Generally, the distribution of males was affected (P ≤ 0.05) by temperature during both seasons, while females were influenced by temperature only in spring. The distribution of both sexes was affected by salinity in spring but not in fall. The distribution of adults sharks (group 3) was more affected by temperature and salinity than that of juveniles sharks (groups 1 and 2). Depth had no effects on the distribution of adults (males or females); however, juveniles belonging to size-group 2 were associated with depth during both spring and fall. High population catches were associated to thermal fronts during all surveys suggesting that these oceanographic structures play an important role as feeding areas for S. guggenheim. A population strategy of spatial segregation by size was proposed and discussed.     

A study of spring-and autumn-spawned herring (Clupea harengus L.) larvae in the Norwegian Coastal Current during spring 1990

An intensive sampling program for yolk-sac herring larvae and microzooplankton was carried out in the main spawning area of Norwegian spring-spawning herring during March to April 1990 (between 62^o and 63^o30'N) to estimate their hatching period and the abundance of copepod eggs and nauplii. Additional investigations were carried out in the Skagerrak area during January-March and on the Norwegian Shelf in May to study the otolith microstructure of the herring larvae. In May both autumn- and spring-spawned herring larvae were found in the samples from the Norwegian Shelf. They were easily distinguished by differences in otolith microstructure. The pattern in increment widths in the otoliths of the autumn-spawned larvae indicated that these larvae had not been transported through the Skagerrak area, but more likely were carried directly from the northern North Sea across the Norwegian Trench and into the Norwegian Coastal Current system. The calculated hatching of the spring-spawned larvae sampled in May occurred significantly later than the observed hatching over the spawning grounds. The results suggest a mismatch between the abundance of first-feeding herring larvae and their prey organisms, resulting in a higher survival of those herring larvae hatching during the latest part of the spawning period. This coincides with a general increase towards the middle of April in the abundance of prey organisms, from 1 to 4 1^_1. There were no differences in otolith microstructure among spring-spawned herring larvae sampled on the shelf in May, indicating that these larvae originated from the same cohort and were well mixed throughout the whole shelf area.     

Subtropical water influences temporal fluctuations of early life stages of Vinciguerria lucetia (Osteichthyes: Phosichthyidae) in the Humboldt Current System (1998–2004)

In the last decades, the fish Vinciguerria lucetia (Garman) has been of important interest to the fisheries sector; nonetheless, the spawning and nursery zones in the Humboldt Current System (HCS) have not yet been defined. By using a temporal series of 23 oceanographic surveys from austral spring of 1998 to autumn 2004 off northern Chile, the spatial and temporal distribution and abundance of eggs and larvae of V. lucetia were studied. The relationships with environmental conditions (sea surface temperature, water column stratification, salinity, dissolved oxygen) were modeled using generalized additive models (GAMs). Seasonal variations in eggs and larval abundances were recorded, and higher abundances were observed in spring and summer, respectively. The main spawning areas were located at approximately between 40 and 80 nautical miles offshore. The largest abundances of V. lucetia eggs were found during spring 2003; however, larval abundances reached the highest values following the strongest ENSO event 1997–98. GAMs predicted that offshore location, sea surface temperature, and the deepening of the oxygen minimum zone, characteristics of the subtropical waters (22–24°C, >34.9, 3–6 ml/L) drove eggs and larval distributions of V. lucetia in the HCS during 1998–2004, toward areas with scarce food availability for larvae. These results suggest that spawning and larval development of this oceanic species occur in oligotrophic waters as a loophole strategy, in order to reduce predation risk during early life stages.     

Differing body size between the autumn and the winter–spring cohorts of neon flying squid (Ommastrephes bartramii) related to the oceanographic regime in the North Pacific: a hypothesis

The neon flying squid (Ommastrephes bartramii), which is the target of an important North Pacific fishery, is comprised of an autumn and winter–spring cohort. During summer, there is a clear separation of mantle length (ML) between the autumn (ML range: 38–46 cm) and the winter–spring cohorts (ML range: 16–28 cm) despite their apparently contiguous hatching periods. We examined oceanic conditions associated with spawning/nursery and northward migration habitats of the two different‐sized cohorts. The seasonal meridional movement of the sea surface temperature (SST) range at which spawning is thought to occur (21–25°C) indicates that the spawning ground occurs farther north during autumn (28–34°N) than winter–spring (20–28°N). The autumn spawning ground coincides with the Subtropical Frontal Zone (STFZ), characterized by enhanced productivity in winter because of its close proximity to the Transition Zone Chlorophyll Front (TZCF), which move south to the STFZ from the Subarctic Boundary. Hence this area is thought to become a food‐rich nursery ground in winter. The winter–spring spawning ground, on the other hand, coincides with the Subtropical Domain, which is less productive throughout the year. Furthermore, as the TZCF and SST front migrate northward in spring and summer, the autumn cohort has the advantage of being in the SST front and productive area north of the chlorophyll front, whereas the winter–spring cohort remains to the south in a less productive area. Thus, the autumn cohort can utilize a food‐rich habitat from winter through summer, which, we hypothesize, causes its members to grow larger than those in the winter–spring cohort in summer.