Age-0 Juveniles Research Articles

Marine heatwaves alter the nursery function of coastal habitats for juvenile Gulf of Alaska Pacific cod

Marine Heatwaves (MHWs) can directly influence survival of marine fishes, particularly for early life stages, including age-0 juveniles during their residence in coastal nursery habitats. However, the ability of nurseries to support high fish densities, optimize foraging and growth, and protect against predators may be altered during MHWs. Gulf of Alaska Pacific cod (Gadus macrocephalus) larval, juvenile, and adult abundances declined dramatically following MHW events in 2014–2016 and 2019. To evaluate coastal nursery function during MHWs, we compared diet composition, recent growth, size, condition, and abundance of age-0 juveniles throughout their first summer before, during, and between MHWs. Diet shifted to larger prey during MHWs, particularly mysids, but diet did not appear to influence growth. We observed faster growth rates during MHWs, yet even when accounting for growth, we could not explain the higher body sizes observed in August during MHWs. Together with lower abundance and the near absence of small fish in the nursery by August during MHWs, these patterns highlight potential for size-selection and a reduced ability of nursery habitats to buffer against environmental variability during MHWs, with only a small number of large “super survivors” persisting through the summer.

The role of temperature on overwinter survival, condition metrics and lipid loss in juvenile polar cod (Boreogadus saida): A laboratory experiment

In the Arctic, winter warming and loss of sea ice pose largely unknown risks to keystone species and the marine ecosystem that they support. Young-of-the-year juvenile polar cod, Boreogadus saida, are an energy-rich forage fish that accumulate high levels of lipid in the summer but retain a relatively small body size during the winter. To address winter bioenergetics and survival, we held age-0 juveniles under simulated winter conditions (food deprived, 24-hr darkness) at a range of four constant temperatures (−1, 1, 3, 5 °C). Our goals were to 1) determine how age-0 polar cod utilize lipid energy in muscle and liver across variable temperatures and durations of food deprivation, 2) understand temperature- and size-dependent impacts on survival and 3) provide energy loss models using multiple condition metrics that are commonly used in fisheries science (lipids, morphometric ratios, body weight). These data have relevance to projecting winter outcomes for polar cod sampled pre-winter, when fish are more easily sampled in the field. As expected, in the absence of food, juvenile polar cod better conserved lipids and survived longer at colder temperatures. There was no negative impact of cold extremes on this pattern; for example, 50% mortality was at 170 days when polar cod were held at −1 °C, compared to only 94 days when they were held at 5 °C. During the first 28 days of simulated winter, polar cod preferentially catabolized triacylglycerols from muscle tissue, then depleted this storage lipid class in their muscle and liver until starvation. Mortality occurred when whole-body lipid concentrations fell below 12.4 mg g−1 wet weight. Temperature-dependent declines in morphometric condition (hepatosomatic index and Fulton's K) and lipid content were parameterized and developed into temperature-dependent condition loss models. Applying a laboratory-based lipid loss model to field-collected polar cod demonstrated that winter survival is highly sensitive to small changes in temperature between −1 and 1 °C when fish are in good condition at the end of the preceding summer. Alternatively, fish in poor summer condition cannot survive winter relying exclusively on stored energy reserves, and will be required to forage throughout the winter. Collectively, these results suggest that lipid-based indices offer a sensitive means of predicting overwintering success for polar cod experiencing climate-driven changes in summer and winter habitats in the Arctic.

Discrimination potential of otolith chemistry to distinguish two parapatric species of flounder (Platichthys) in the Baltic Sea

Baltic Sea flounder were recently split into two species, the offshore spawner Platichthys flesus and coastal spawner Platichthys solemdali. The two species can only be distinguished based on egg and sperm characteristics and via genetic analyses, which limits the species identification methods of larvae and juveniles to molecular techniques. We investigated whether otolith chemistry could be used as an additional tool to identify flounder to species level. We tested for species-specific differences in otolith multi-elemental signatures and spatial consistency of those differences for the early life stages of flounder in three areas of the central Baltic Sea (ICES SD 24–28), where the distribution of both species overlaps. Otolith chemistry signatures obtained through maternal transfer (i.e. core chemistry) and signatures that reflect the post-hatching phase were not significantly different between species. Species-specific differences at the sub-regional scale were only found for the Latvian coastal survey area for multiple elements (Ba, Cu, Mg, Pb, Sr and Zn), but were insufficiently distinct for reliable species discrimination. Geographic classification of age-0 juveniles to survey area was more successful than classification to species, which was reflected by a spatial trend in otolith Sr:Ca that followed the salinity gradient and higher Mn:Ca and I:Ca for Latvian individuals. Otolith chemistry of early life flounder from the Baltic Sea reflects spatial variability in environmental conditions but does not differentiate between the two flounder species in sympatric habitats.

Variation in predator diet and prey size affects perceived impacts to salmon species of high conservation concern

Management of protected species is difficult when objectives include the recovery of both predator and its prey. Ideally, identifying trade-offs between competing objectives involves evaluating management alternatives with a quantitative model that integrates information on both species, but data are often limited. We used new predator diet data and simulation modeling to update our understanding of seal predation on juvenile Chinook (Oncorhynchus tshawytscha) and coho salmon (Oncorhynchus kisutch) in the Puget Sound. Under prey size assumptions used by previous studies, estimates suggest predation could be a significant source of mortality for Chinook salmon (mean: 37% of ocean age-0 juveniles), but varied considerably among years (range of median estimates: 22%–49%). However, when we estimated prey size from otoliths recovered from seal scats, the impact (numbers of fish consumed) decreased by 71%. Predation on coho salmon was estimated to be relatively low under both scenarios (6%–8% of ocean age-0 juveniles) with a 21% decrease in consumption using otolith-derived prey size. Our analysis highlights the importance of updating model inputs and re-evaluating assumptions of multi-species models used for ecosystem-based fisheries management.

Habitat use by early life stages of the re-established striped bass and conspecific fish species along the St. Lawrence estuary

The St. Lawrence estuary (SLE) supports important nursery habitats for many fish species, including the newly re-established population of striped bass (Morone saxatilis).The present study aims to describe pelagic (5 m isobaths) and littoral (<1.5 m-deep) nearshore habitats of the SLE and to document their use by fish larvae and age-0 juveniles, especially for striped bass. From June to September 2014, 162 pelagic and 188 littoral stations were characterized in terms of biophysical conditions (turbidity, salinity, temperature, dissolved oxygen and chlorophyll-a concentration), abundance of main preys and fish assemblages. Striped bass larvae identification was confirmed with genetic analyses. In the pelagic and littoral zones, four estuarine habitats were defined based on turbidity and salinity: an upstream freshwater section (UP), an oligohaline (O-ETM) and a mesohaline (M-ETM) estuarine turbidity maximum zone, and a downstream polyhaline section (DOWN). Our results emphasized the spatial partitioning of fish early life stages among contrasting habitats, potentially to minimize interspecific competition. In the SLE, striped bass is using the wide spectrum of habitats available. At the onset of striped bass development, the UP and the O-ETM were the main nursery habitats, which is consistent with the known location of spawning grounds. Results suggest that the O-ETM provided the most suitable conditions for the species with optimal physical properties and abundant food resources. Later in the season, the M-ETM provided a favourable trade-off habitat offering a less optimal physical environment although potential for new prey.

Introgression of hatchery rainbow trout in naturalized steelhead populations of western Lake Superior

Fisheries managers and anglers in Minnesota have long been concerned that reproduction in the wild by a hatchery strain of rainbow trout (Oncorhynchus mykiss), called kamloops, could reduce the fitness of naturalized steelhead (migratory rainbow trout) populations in Lake Superior. A previous study found no evidence of kamloops introgression, but a new evaluation is warranted in light of continued stocking and genetic advancements over the past 25 years. We used genotypes from 10 microsatellite DNA loci to assess kamloops ancestry in adult and age-0 rainbow trout samples from 40 streams in Minnesota, one stream in Wisconsin, and two adult broodstocks. We also evaluated genetic population structure for impacts of kamlooops introgression. Overall, the average estimated kamloops ancestry was 8% (range 2–44%) in age-0 juveniles and 4% (1–19%) in adults. The average percentages of kamloops descendants in age-0 and adults were 8% (0–54%) and 5% (0–40%), respectively. Kamloops descendants were found in many Minnesota streams and in Wisconsin’s Bois Brule River, but were most prevalent in streams near the source hatchery, near recent stocking locations, and in lower reaches of longitudinally sampled streams. Feral steelhead broodstock had 9% and 6% kamloops descendants in two years while captive broodstock had no kamloops descendants. Minnesota’s populations showed little spatial genetic structure but were distinct from the Bois Brule population and from kamloops, which indicated that kamloops introgression had not substantially altered structure. Kamloops introgression into Minnesota steelhead populations and impacts on other jurisdictions contributed to the decision to discontinue the kamloops stocking program.

Sub-annual cohort representation among young-of-the-year recruits of the western stock of Atlantic bluefin tuna

The western stock of Atlantic bluefin tuna receives recruits principally from spawning habitats in the Gulf of Mexico although the recent discovery of bluefin tuna larvae in the western North Atlantic Slope Sea calls into question whether there is another key source of recruits to the western stock. We tested whether age-0 juveniles occurring in the US Mid-Atlantic Bight during the late summer might originate from Slope Sea spawning by estimating the hatch date distribution based on otolith microstructural analysis given larval production occurs approximately two months later in the Slope Sea (late June to early August) than in the Gulf of Mexico (early April to mid-June). Following a review of the literature on ageing methodology for Thunnini species, we attempted trials on both sectioned and whole un-sectioned otoliths, and adopted the latter based on higher precision levels. Estimated ages ranged from 122 to 175 (days post hatch) for fish ranging 23.2–36.3 cm fork length. Hatch dates ranged 20 March to 12 May 2010, exhibiting a unimodal distribution. Hatch dates mainly occurred in early-April 2010, which corresponded to the spawning and larval production period for the Gulf of Mexico. Estimated growth rate (size-at-age) was 1.1 mm d−1. Thus, we failed to observe evidence for recruitment from the newfound Slope Sea spawning region, albeit our sample represents only a single year and region. More conclusive inferences on recruitment from the Slope Sea spawning will depend on a greater sample of age-0 juveniles sampled across years or molecular approaches that can identify recruits in older juveniles and adults.

Kokanee–sockeye salmon hybridization leads to intermediate morphology and resident life history: implications for fisheries management

Stocking programs designed to return extirpated species to their historical range have become increasingly prevalent, punctuating the need to better understand the risks posed to recipient ecosystems. Here, we investigated the genetic and biological consequences of an anadromous sockeye salmon (Oncorhynchus nerka) stocking program in Skaha Lake, British Columbia, where substantial levels of hybridization and introgression with the native freshwater resident ecotype (kokanee) have been detected. We genetically assigned 543 individuals (adult spawners, age-0 juveniles) to estimate stock proportions (pure-stock sockeye, pure-stock kokanee, or hybrid) between 2010 and 2017, with a subset undergoing otolith microchemistry analysis to determine migratory life history and maternal ancestry. The proportion of hybrid spawners varied from 5% to 20% across sampling years, while that of hybrid age-0 juveniles remained relatively constant (∼11%). Hybrid spawners exhibited intermediate size relative to pure stocks, with the vast majority being nonanadromous (92%) and of resident maternal ancestry (76%). Our results provide empirical support for previously hypothesized mechanisms of hybridization between O. nerka life-history forms and underscore the importance of continued monitoring of stocking programs to quantify long-term fitness impacts of introgression and refine management strategies.

Spatiotemporal dynamics of predators and survival of marine fish early life stages: Atlantic cod (Gadus morhua) in the North Sea

Predation is one of the principle factors regulating the survival of early-life stages and recruitment success of marine fishes. Although challenging, it is important to understand how predation mortality varies in space and how this spatial variability affects the temporal dynamics of fish recruitment. We investigated the spatiotemporal variability in the magnitude of predation mortality and survival of Atlantic cod (Gadus morhua) in the North Sea by combining a Lagrangian individual-based model of cod early-life stages (eggs, larvae and age-0 juveniles) with an Eulerian representation of its fish predators based on field data collected quarterly from 1991 to 1997. The mean and variance (spatial and inter-annual) in survival of cod were different between simulations using homogeneous and heterogeneous spatial distributions of predators. Survival of cod through its early-life period was 1 order of magnitude higher in simulations using heterogeneous versus homogenous predator distributions, mainly due to reduced mortality of eggs and larvae in the former simulation. Our model predicted markedly different year-to-year changes in cod survival, when heterogeneous predator fields were implemented. In the heterogeneous simulation, Atlantic herring (Clupea harengus) and grey gurnard (Eutrigla gurnadus) emerged as the most important predators comprising, respectively, 68% and 13% of the predation mortality caused by 7 predators included in our study. A low (<0.1%) survival was predicted for cod progeny originating from known spawning grounds in the central and south-eastern North Sea while considerably higher survival (between 0.8 and 1%) was predicted for northern spawning areas and in the Southern Bight. Numerical sensitivity experiments indicated that seasonal and annual changes in the spatial distribution of predators were more important to the inter-annual variability in the modelled cod survival (52% of the mean survival) than annual changes in the biomass of predators (23%) and the variability of the North Sea hydrography (19%) with the relative importance of these factors depending on the life stage of cod. We argue that including spatial dynamics of predators will advance the predictive capacity of fish early-life stage models enabling more robust, science-based advice for spatially-explicit fishery management and marine spatial planning.

American alligators are predators of Paddlefish: An undocumented predator-prey linkage affecting Paddlefish population dynamics

It has been suggested that adult Paddlefish Polyodon spathula have no natural predators with the exception of humans and few heterospecific freshwater fishes which tend to target only early life stages (i.e., larvae, age-0 juveniles). However, we photographically documented another natural predator, the American alligator. The predation event took place in a small waterbody (0.8 ha) over a 45-minute period during which the alligator frequently transported the Paddlefish onto land, and retracted back into the water. This is the second event of its kind documented in this small river system. From a capture-recapture study, we have identified over 100 fish in the pool, 50 of which were surgically implanted with an acoustic transmitter for continuous monitoring of emigration. However, two have unexplainably disappeared, presumably from predation. Given the two known alligator mortality events, and two potential additional events suggested by transmitter disappearance, we presume adult Paddlefish predation may be more common than previously thought. Although adult Paddlefish were thought to have no natural predators, this newly documented predator-prey linkage provides support that alligator predation on Paddlefish may occur more frequently in southeastern waterbodies than previously believed, particularly when Paddlefish are in high abundance or in relatively small systems such as ours.

Complex post-larval dispersal processes in Atlantic cod revealed by age-based genetics and relatedness analysis

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 556:237-250 (2016) - DOI: https://doi.org/10.3354/meps11819 Complex post-larval dispersal processes in Atlantic cod revealed by age-based genetics and relatedness analysis John B. Horne1,*, Ian R. Bradbury2, Ian G. Paterson1, David Hardie3, Jeffrey Hutchings1,4, Benjamin J. Laurel5, Paul V. R. Snelgrove6, Corey J. Morris2, Robert S. Gregory2, Paul Bentzen1 1Marine Gene Probe Laboratory, Department of Biology, Dalhousie University, 1355 Oxford Street, Halifax, NS B3H 4R2, Canada 2Department of Fisheries and Oceans Canada, 80 E. White Hills Road, St. John’s, NL A1C 5X1, Canada 3Department of Fisheries and Oceans Canada, Dartmouth, NS B2Y 4T3, Canada 4Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo, Oslo 0316, Norway 5Fisheries Behavioral Ecology Program, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, Hatfield Marine Science Center, Newport, OR 97365, USA 6Department of Ocean Sciences and Biology Department, Memorial University of Newfoundland, St. John’s, NL A1C 5S7, Canada *Corresponding author: john.horne@gmail.com ABSTRACT: Population connectivity among adult marine organisms is often attributed to dispersal during the egg/larval stage. However, post-larval dispersal may also influence connectivity, particularly when juvenile nursery habitats are separated from adult spawning habitats. Here we used age-based population genetics and kinship analysis to explore changes in population connectivity across life-history stages in Atlantic cod Gadus morhua. We genotyped 364 adult cod from the northwest Atlantic and 671 age-0 juveniles from 18 sites around eastern Newfoundland, with 72 and 15 microsatellite loci, respectively. Adult cod genotypes exhibited more population structure than was detected in juveniles across similar spatial scales. Both age classes had similar allelic diversities, but juveniles exhibited less genetic linkage and fewer departures from Hardy-Weinberg expectations than adults at the same loci. We detected significant kinship relationships in adult cod only within sampling locations, but 1 putative pair of juvenile kin was separated by >500 km. Collections of adults also displayed higher group relatedness compared to juveniles. Genetic differences between age classes are likely due to a combination of non-random mortality and non-random sorting of admixed juveniles into different adult habitats. Many studies overlook post-larval dispersal as a factor of marine population connectivity, but pre-adult relocation may be demographically and adaptively significant, in cod and other species. KEY WORDS: Gadidae · Kinship · Newfoundland · Marine dispersal · Fisheries · Northwest Atlantic Full text in pdf format Supplementary material PreviousNextCite this article as: Horne JB, Bradbury IR, Paterson IG, Hardie D and others (2016) Complex post-larval dispersal processes in Atlantic cod revealed by age-based genetics and relatedness analysis. Mar Ecol Prog Ser 556:237-250. https://doi.org/10.3354/meps11819 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 556. Online publication date: September 08, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Modeling connectivity of walleye pollock in the Gulf of Alaska: Are there any linkages to the Bering Sea and Aleutian Islands?

We investigated the connectivity of walleye pollock in the Gulf of Alaska (GOA) and linkages to the Bering Sea (BS) and Aleutian Island (AL) regions. We used a spatially-explicit Individual-based model (IBM) coupled to 6 years of a hydrodynamic model that simulates the early life history of walleye pollock in the GOA (eggs to age-0 juveniles). The processes modeled included growth, movement, mortality, feeding and the bioenergetics component for larvae and juveniles. Simulations were set to release particles on the 1st of the month (February to May) in fourteen historical spawning areas in the GOA up to the 1st of September each year. Model results reproduced the link between the Shelikof Strait spawning area and the Shumagin nursery region for March and April spawners, besides other Potential Nursery Areas (PNAs) found in the GOA. A prominent finding of this study was the appearance of the BS as important PNAs for several GOA spawning grounds, which is supported by a consistent flow into the BS through Unimak Pass. The simulations showed the highest density of simulated surviving pollock in the western Bering Sea (WBS) region with the lowest coefficients of variation of the whole domain. Three spawning sectors were defined, which aggregate multiple spawning areas in the eastern (EGOA), central (CGOA) and western Gulf of Alaska (WGOA). A connectivity matrix showed strong retention within the CGOA (25.9%) and EGOA (23.8%), but not in the WGOA (7.2%). Within the GOA, the highest connectivity is observed from EGOA to CGOA (57.8%) followed by the connection from CGOA to WGOA (24.3%). Overall, one of the most prominent connections was from WGOA to WBS (62.8%), followed by a connection from CGOA to WBS (29.2%). In addition, scenarios of shifting spawning locations and nursery sectors of GOA, BS and AL are explored and implications for walleye pollock stock structure hypotheses are discussed.

Assessment of resource selection models to predict occurrence of five juvenile flatfish species (Pleuronectidae) over the continental shelf in the western Gulf of Alaska

According to the nursery size hypothesis, flatfish recruitment is constrained by nursery area. Thus, if resource selection models can be shown to accurately predict the location and geographic extent of flatfish nursery areas, they will become important tools in the management and study of flatfish population dynamics. We demonstrate that some resource selection models derived previously to predict the presence and absence of juvenile flatfishes near shore were applicable to the broader continental shelf. For other age-species groups, derivation of new models for the continental shelf was necessary. Our study was conducted in the western Gulf of Alaska (GoA) during October 2011 on four groups of age-0 juvenile flatfishes: Pacific halibut (Hippoglossus stenolepis), arrowtooth flounder (Atheresthes stomias), northern rock sole (Lepidopsetta polyxystra), and flathead sole (Hippoglossoides elassodon); and three groups of age-1 juvenile flatfishes: northern rock sole, flathead sole, and yellowfin sole (Limanda aspera). Sampling occurred at 33 sites across the continental shelf. Fish were collected using a 3-m beam trawl, and a midwater trawl. Environmental data were collected on sediment composition and water temperature and depth. Many of the age-species groups co-occurred in the Shumagin and Barnabus sea valleys; however, age-0 arrowtooth flounder occurred at more locations than other juveniles, perhaps due to a relatively broad tolerance of environmental conditions and to the utilization of midwater habitat. Thus, the large nursery area of arrowtooth flounder may be one reason why they are currently the most abundant GoA flatfish. In fact, among all species, mean recruitment at age 3 increased with the percent occurrence of age-0 juveniles at the 33 sites, a proxy for relative nursery area, in accordance with the nursery size hypothesis, suggesting that mean recruitment among GoA flatfishes is structured by nursery size.

Vertical segregation of age-0 and age-1+ polar cod (Boreogadus saida) over the annual cycle in the Canadian Beaufort Sea

The offshore marine ecosystem of the Canadian Beaufort Sea faces the double pressure of climate change and industrialization. Polar cod (Boreogadus saida) is a pivotal forage species in this ecosystem, accounting for 95 % of the pelagic fish assemblage. Its vertical distribution over the annual cycle remains poorly documented. Hydroacoustic records from 2006 to 2012 were analysed to test the hypothesis that age-0 polar cod segregate vertically from larger congeners. Trawls and ichthyoplankton nets validated the acoustic signal. Fish length, weight, and biomass were estimated from new regressions of target strength and weight on standard length. Polar cod were vertically segregated by size in all months, with small age-0 juveniles in the epipelagic (<100 m) layer and larger age-1+ deeper in the water column. From December to March, the biomass of age-1+ peaked in a mesopelagic layer between 200 and 400 m. With increasing irradiance from April to July, the mesopelagic layer deepened and extended to 600 m. Starting in July, age-0 polar cod formed an epipelagic scattering layer that persisted until November. From September onward, age-0 left the epipelagic layer to join small age-1+ in the upper mesopelagic layer. Low biomass in the mesopelagic layer from February to September likely resulted from large polar cod settling on the seafloor to avoid diving marine mammals. Longer ice-free seasons, warmer sea-surface temperatures, or an oil spill at the surface would likely impact epipelagic age-0, while mesopelagic age-1+ would be vulnerable to an eventual oil plume spreading over and above the seafloor.

Ontogenetic and spatial variability in trophic biomarkers of juvenile saffron cod (Eleginus gracilis) from the Beaufort, Chukchi and Bering Seas

Climate models indicate the Arctic will undergo dramatic environmental change with forecasted increases in temperature and river runoff. Saffron cod (Eleginus gracilis) is abundant in nearshore waters and appears in the diet of many Arctic sea birds and marine mammals; however, little is known about its early ecology and consequently how they might be affected by environmental changes. We aimed to characterize the mechanisms of spatial and ontogenetic variation in trophic biomarkers (lipid classes, fatty acids and bulk C and N stable isotopes) of saffron cod from the Western Arctic, Chukchi and Bering Seas. Size-standardized analyses showed a significant difference in lipid condition metrics and trophic biomarkers as a function of survey location. Both ontogeny and sampling location played an important role in determining lipid stores with elevated levels in both small offshore juveniles ( 75 mm). Higher lipid storage in Arctic juveniles was associated with elevated levels of diatom fatty acid markers, but not with nearshore carbon input. Increased lipids were found in age-1 juveniles from Prudhoe Bay in the Western Beaufort that were feeding at a lower trophic level than similarly sized age-0 juveniles from surface trawls in the Bering Sea. The use of otolith annuli revealed two discrete patterns of growth that help explain the trade-offs between energy storage and rapid growth that diverge between the Arctic and Bering Sea. Laboratory temperature-growth experiments confirmed that saffron cod have a eurythermal growth response and are able to store excess lipids at temperatures as high as 20 °C.

A hierarchical Bayesian model for embedding larval drift and habitat models in integrated life cycles for exploited fish

This paper proposes a hierarchical Bayesian framework for modeling the life cycle of marine exploited fish with a spatial perspective. The application was developed for a nursery-dependent fish species, the common sole (Solea solea), on the Eastern Channel population (Western Europe). The approach combined processes of different natures and various sources of observations within an integrated framework for life-cycle modeling: (1) outputs of an individual-based model for larval drift and survival that provided yearly estimates of the dispersion and mortality of eggs and larvae, from spawning grounds to settlement in several coastal nurseries; (2) a habitat suitability model, based on juvenile trawl surveys coupled with a geographic information system, to estimate juvenile densities and surface areas of suitable juvenile habitat in each nursery sector; (3) a statistical catch-at-age model for the estimation of the numbers-at-age and the fishing mortality on subadults and adults. The approach provided estimates of hidden variables and parameters of key biological significance. A simulation approach provided insight to the robustness of the approach when only weak data are available. Estimates of spawning biomass, fishing mortality, and recruitment were close to the estimations derived from stock-assessment working groups. In addition, the model quantified mortality along the life cycle, and estimated site-specific density-dependent mortalities between settled larvae and age-0 juveniles in each nursery ground. This provided a better understanding of the productivity and the specific contribution of each nursery ground toward recruitment and population renewal. Perspectives include further development of the modeling framework on the common sole and applications to other fish species to disentangle the effects of multiple interacting stress factors (e.g., estuarine and coastal nursery habitat degradation, fishing pressure) on population renewal and to develop risk analysis in the context of marine spatial planning for sustainable management of fish resources.

Disentangling bottom-up and top-down effects on survival during early ocean residence in a population of Chinook salmon (Oncorhynchus tshawytscha)

We evaluated the relative importance of “bottom-up” (production-limited) and “top-down” (predator-mediated) processes during early marine residence in a population of Chinook salmon (Oncorhynchus tshawytscha) from the upper Columbia River, USA. We examined length, mass, and condition index of age-0 juveniles collected in the ocean during June and September across 11 years in relation to conditions in the river, estuary, and coastal ocean and to future adult returns. Characteristics of juveniles in September, but not June, were related to adult returns. During years when coastal waters were relatively cool and productive, juveniles captured in September displayed relatively low condition and reduced otolith growth compared with years when coastal waters were relatively warm and unproductive; this contrast indicates that top-down effects such as selective mortality or competition are important during early marine residence. Key physical (river plume volume during emigration) and biological (condition) variables and their interaction accounted for &gt;95% of the variation in adult returns. Future research should focus on evaluating predators and competitors and understanding how river plume structure influences survival.

Factors contributing to variability in larval ingress of Atlantic menhaden, Brevoortia tyrannus

Annual recruitment levels of age-0 juvenile Atlantic menhaden to Chesapeake Bay, which historically supported >65% of coastwide recruitment, have been consistently low since the 1980s. Diminished larval supply to the Bay is one hypothesized explanation. In a three-year ichthyoplankton survey at the Chesapeake Bay mouth, abundance of ingressing larvae varied nine-fold among years. Larvae were most abundant in 2007–2008 and less abundant in 2005–2006 and 2006–2007. High month-to-month variability in larval concentrations was attributable primarily to seasonality of occurrences. There was no defined spatial pattern in distribution of larvae across the 18-km-wide Bay mouth, but larvae at the south side were longer and older on average than larvae at the middle and north side. Environmental variables measured at the times of larval collections were not correlated consistently with temporal and spatial variability in abundance of larvae at ingress, highlighting complexity and suggesting that abundance may be controlled by processes occurring offshore during the pre-ingress phase. Moreover, the substantial differences in inter-annual abundances of larvae at the Bay mouth were not concordant with subsequent abundances of age-0 juveniles in the three survey years, indicating that important processes affecting recruitment of Atlantic menhaden operate after ingress, during the larval to juvenile transition stage.

Differences in juvenile trophic niche for two coastal fish species that use marine and estuarine nursery habitats

In coastal regions, age-0 juveniles of many fish species are capable of recruiting to either marine or estuarine nursery habitats, yet the ecological consequences for cohorts that use marine versus estuarine nurseries is poorly understood. In the present study, stable isotope (δ13C, δ15N) and stomach contents data were used to compare trophic ecology associated with differential habitat use for age-0 bluefish Pomatomus saltatrix and bay anchovy Anchoa mitchilli from 2 habitats: Maryland’s (USA) inner continental shelf (shelf) and lower Chesapeake Bay (estuary). Bluefish occupied equivalent trophic positions (approx. 4.0 to 4.2; δ15Nand diet-based estimates) as tertiary consumers in the shelf and estuary. In contrast, bay anchovy were secondary-tertiary consumers with trophic position estimates of 3.5 to 3.8 in the shelf and 3.5 in the estuary. A C:N ratio proxy for lipid content was higher in the shelf cohorts for both species, and weight-at-length was also higher for shelf bay anchovy than estuarine bay anchovy. Estuarine cohorts of both species occupied a larger isotopic niche (i.e. convex hull area), yet 2-source mixing model results indicated that estuarine cohorts derived >80% of their biomass from pelagic food webs alone. Conversely, shelf cohorts more equitably integrated pelagic (bluefish: 45 ± 7% [SD], bay anchovy: 46 ± 12%) and benthic food webs. The present study indicates that the juvenile trophic niche of these species can vary significantly across habitats and provides initial evidence that cohorts recruiting to Maryland’s shelf can realize more diverse (i.e. utilization of multiple food webs) or superior (i.e. increased condition — bay anchovy only) foraging conditions than cohorts recruiting to proximal estuaries.

Habitat utilization, feeding, and growth of wild spotted halibut Verasper variegatus in a shallow brackish lagoon: Matsukawa-ura, northeastern Japan

Habitat utilization, feeding, and growth of a rare pleuronectid flatfish, spotted halibut Verasper variegatus, were examined in a brackish lagoon in northeastern Japan: Matsukawa-ura. The distribution and date–length data of spotted halibut collected mainly from beam-trawl samplings during 1983–2008 indicated that age-0 juveniles [n = 25, 6.0–18.0 cm total length (TL)] and older spotted halibut (n = 71, 13.8–43.0 cm TL) inhabited almost the entire northern part of Matsukawa-ura. Comparative distribution surveys of spotted halibut, stone flounder, and marbled flounder during 1985–1989 revealed similar distribution patterns of these flatfishes; the highest densities were found around the central part of Matsukawa-ura, although significantly lower abundance was detected for spotted halibut. Linear growth equations of age-0 juveniles demonstrated that spotted halibut were able to achieve high growth after June, probably because abundant prey (e.g., mysids and gammarids) and suitable physical conditions (warmer water temperature and lower salinity) were present. The main prey items shifted from various crustaceans including mysids, caridean shrimps, and anomurans to brachyura Hemigrapsus spp. (≥20 cm TL). Our study shows that spotted halibut use the shallow brackish lagoon as an important nursery for juveniles, and also as a feeding ground for young and adults.