Leucoraja Ocellata Research Articles

Positional and ontogenetic variation in vertebral centra morphology in five batoid species

An increasing number of studies on elasmobranchs have shown that band-pair counts in vertebral centra do not accurately reflect age. Research in sharks has indicated that the number of band pairs vary with body size and that centrum morphology is related to structural needs. A study of this kind has not been undertaken on batoids; thus, we examined the relationship between band-pair deposition and morphology of centra along the vertebral column, and ontogenetically, for five batoid species (little skate, Leucoraja erinacea, winter skate, Leucoraja ocellata, barndoor skate, Dipturus laevis, Atlantic stingray, Dasyatis sabina, and round ray, Urobatis halleri). Centrum morphology and band-pair count varied along the vertebral column in all individuals of all species, except in young of the year. Variation in band-pair counts among centra within individuals supports the hypothesis that band-pair formation is related to somatic growth and body shape rather than to an annual cycle.

P.170 Polymorphisms of serotonergic system genes and their association with remission in patients with depression

Due primarily to regulatory factors, skates (family Rajidae) account for nearly half the total bycatch discarded during commercial fishing operations in the U.S. portion of the Northwest Atlantic Ocean. In this region the New England scallop dredge fishery has the second highest skate discard rate; however, no information regarding their resiliency to interaction with this gear type exists. To gain insight into species-specific mortality rates, 295 tows were conducted throughout six research cruises (2012–2013 fishing season), with a total of 4216 skates (little skate, Luecoraja erinacea, winter skate, Leucoraja ocellata, and barndoor skate, Dipturus laevis) evaluated and scored using reflex impairment and injury (i.e., overt physical trauma) indicators. To quantify mortality rates associated with these indicators, 334 skates were retained in an on-deck refrigerated flow-through seawater system and monitored every six-hours until the end of a trial. This study also assessed the effect of fishing conditions/practices and individual traits on post-release mortality. Data suggest that species-specific differences exist wherein winter skate are likely the most resilient (45.4% mortality) to capture and handling, little skate have a moderate capacity to deal with these pressures (62.7% mortality), and barndoor skate appear to be the most susceptible (99.9% mortality). In addition, little and winter skate mortality rates were both influenced by tow duration, while fishing depth and temperature gradient had an effect on little skate and air exposure on winter skate.

Release of unwanted flat-bodied fish from a horizontal-bar grid system as revealed through comparative fishing trials and underwater video observations

Grid systems separate fish species primarily through physical means: fish size and body shape. On Georges Bank off the northeast USA, many species of flounders are overfished, and their catch needs to be reduced. Flat-bodied skates are also often discarded. We tested a European style horizontal-bar grid system to reduce these flat-bodied low quota species in a trawl targeting the haddock (Melanogrammus aeglefinus), whose population is robust. The grid system consisted of 4 grid sections, two on each side, with horizontal bars 70 mm apart. The alternating tow method was used to compare the catch characteristics between a trawl with a grid section and the same trawl without a grid section. A video camera was used to observe fish escape in the grid section. The results indicate that the grid system reduced the flounder catch rate (mainly winter flounder, Pseudopleuronectes americanus) by 51.3%, and skates (mainly winter skate, Leucoraja ocellata and little skate, Leucoraja erinacea) by 29.4%, while there were no differences in the catch of Atlantic cod (Gadus morhua). While haddock was reduced by 37% by weight, the reduction was primarily small undersized individuals. There was no reduction in large haddock greater than 50 cm. Video observations indicates that flounders and cod mainly escaped from the bottom half of the grid while haddock were from the top half. These finding suggest that this system reduced low quota flounders, as well as discarded skates and small haddock, while retaining Atlantic cod and large size haddock. Underwater observations indicate that differential spacing (narrower on the top and wider on the bottom) may improve the system performance by releasing more flat-bodied fish, that were observed to escape from the bottom part of the grid, while retaining more haddock, which typically escape from the top part of the grid.

Risk of extinction of a unique skate population due to predation by a recovering marine mammal

Benefitting from reduced harvesting and an end to culling, many marine mammals are now recovering from past overexploitation. These recoveries represent important conservation successes but present a serious conservation problem when the recovering mammals are predators of species of conservation concern. Here, we examine the role of predation by recovering grey seals (Halichoerus grypus) in the near-extinction of a unique skate population in the southern Gulf of St. Lawrence (sGSL) in Atlantic Canada. Winter skate (Leucoraja ocellata) in the sGSL are distinct from winter skate elsewhere and may represent an endemic species. Their adult abundance has declined by 98% since 1980, and these skates are now detectable in only a small fraction of their former range. Population modeling indicates that the ongoing collapse of this population is due to increases in the natural mortality of adults. Based on model projections, this population would be extinct by mid-century if its current rate of productivity were to persist. A second population model incorporated predation by grey seals. Model estimates of skate consumption by seals were consistent with historical and recent estimates of the contribution of skates to grey seal diets. The estimated consumption accounted for the increases in the natural mortality of adult skates. A Type III functional response for grey seals preying on winter skate emerged from the model results. This indicates that, when skate abundance is very low, grey seals are expected to switch to alternate prey, resulting in declines in the mortality of skates due to predation. Consequently, contrary to projections at current productivity, winter skate are expected to be trapped at very low abundance in a "predator pit" instead of declining to extinction. Nonetheless, extinction risk would remain very high at the very small population size in the predator pit. Our results emphasize the need for an ecosystem-based approach to the management of living resources in this ecosystem.

A Tale of Two Skates: Comparative Phylogeography of North American Skate Species with Implications for Conservation

Genomic data can provide novel insights into the natural history of oceanic species. These data can inform the management of vulnerable and slow-maturing species by estimating population structure, rates of migration, and the distribution of genetic diversity. In this study we focus on two protected elasmobranch species, the Winter Skate, Leucoraja ocellata, and the Little Skate, L. erinacea. We use genome-wide SNPs to estimate population structure, and quantify migration and genetic diversity among both species from four sampling localities across the Atlantic coast of North America. We find that species of Leucoraja are generally isolated by distance, although we infer some fine-scale population structure. Specifically, estimates of effective migration infer fine-scale population structure in L. ocellata between the northern sites of Georges Bank and the Mid-Atlantic sampling sites, whereas L. erinacea shows no evidence of population genetic structure in any analyses. We also found that genetic diversity is concentrated in the central sites of Georges Bank and the Mid-Atlantic Bight for L. ocellata, but is reduced at these two sites in L. erinacea, suggesting opposite distributions of genetic diversity between species. Thus, genomic data suggest that while species of Leucoraja lack discrete population structure, they likely employ only mid-range dispersal. These findings correspond to ecological studies that have found eco-physiological differences between embryonic and juvenile Leucoraja from different localities. Taken together, small-bodied skate research emphasizes the importance of local adaptive plasticity for marine species, even without population genetic structure. Conservation strategies should focus on managing the portions of the Atlantic coast considered most vital to reproduction of Leucoraja, but should not recognize multiple populations across their range.

Species composition and habitat preferences of the nearshore fish fauna of Bonne Bay, Newfoundland

BackgroundDetermining biodiversity baseline is the first step toward establishing species monitoring and conservation programs. In this study we report on a 10-year survey of the fish fauna of Bonne Bay, a fjord surrounded by the Gros Morne National Park on the west coast of Newfoundland, Canada. The objectives of our study were: 1) to determine the fish fauna of Bonne Bay using standardized sampling methods; 2) to gather information on the habitats of fishes of conservation concern; and 3) to provide baseline information on Bonne Bay as a potential candidate for a National Marine Conservation Area (NMCA).MethodsA survey of the fish fauna of the inner Bonne Bay was conducted each summer from 2002 to 2011 at multiple sites representing a range of fish habitats within the fjord. Sampling gears included two types of beach seine, gillnets with various mesh sizes and a bottom trawl. Species composition was statistically compared across sites within the fjord.ResultsWe collected and identified 29 fish species from 17 families. Fish assemblages comprised anadromous, estuarine, and marine fish species, including a late-maturing type of winter skate (Leucoraja ocellata) that is rarely found in the adjacent waters of the northern Gulf of St. Lawrence. Similarity in species composition across sites reflected salinity, substrate composition, and presence of eelgrass (Zostera marina), but not the geographic distance between sites.ConclusionsBonne Bay’s adjacency to a United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage Site, its diverse fish fauna that includes several species of conservation concern, its potential for education and enjoyment, and its stewardship by local people are suggestive of the future candidacy as a NMCA. The data presented here will help managers assess the potential of Bonne Bay as a National Marine Conservation Area.

Feeding Interactions between Fishes in a Coastal Ecosystem in the Southern Gulf of St. Lawrence, Atlantic Canada

AbstractDiet composition, diet similarity, and major predator–prey interactions between the 15 most abundant demersal and pelagic fishes (N = 12,163 stomachs) during the period 1999–2003 were described for Northumberland Strait, a semi‐enclosed, marine coastal ecosystem. Of the five pelagic species, Rainbow Smelt Osmerus mordax ate benthic prey (shrimps and polychaetes); Atlantic Herring Clupea harengus and American Shad Alosa sapidissima consumed small copepods and crab zoeae; and Atlantic Mackerel Scomber scombrus and Alewives Alosa pseudoharengus ate small copepods and moderate amounts of benthic prey and small fishes. Three demersal species were strongly piscivorous but with minimal diet overlap: Sea Ravens Hemitripterus americanus consumed small benthic fishes (especially flounders); Winter Skate Leucoraja ocellata consumed sand lances Ammodytes sp. and Rainbow Smelt; and White Hake Urophycis tenuis consumed Atlantic Herring and Atlantic Mackerel. Cluster analysis revealed seven feeding guilds, of which four contained a single species; consequently, the potential for compensation of predatory function was nonexistent for these guilds. The three multispecies guilds comprised consumers of (1) small copepods and crab zoeae; (2) small benthic invertebrates and polychaetes; and (3) shrimps and crabs. Based on diets, the principal route of energy transfer seemed to be the benthic–detrital pathway; however, confirmation requires representative abundance estimates for the various fishes. Moreover, abundances of many fish species have changed (i.e., most have decreased) since this study was completed, with unknown consequences for food web structure and functioning.

The survival of rajids discarded in the New England scallop dredge fisheries

Abstract Due primarily to regulatory factors, skates (family Rajidae) account for nearly half the total bycatch discarded during commercial fishing operations in the U.S. portion of the Northwest Atlantic Ocean. In this region the New England scallop dredge fishery has the second highest skate discard rate; however, no information regarding their resiliency to interaction with this gear type exists. To gain insight into species-specific mortality rates, 295 tows were conducted throughout six research cruises (2012–2013 fishing season), with a total of 4216 skates (little skate, Luecoraja erinacea, winter skate, Leucoraja ocellata, and barndoor skate, Dipturus laevis) evaluated and scored using reflex impairment and injury (i.e., overt physical trauma) indicators. To quantify mortality rates associated with these indicators, 334 skates were retained in an on-deck refrigerated flow-through seawater system and monitored every six-hours until the end of a trial. This study also assessed the effect of fishing conditions/practices and individual traits on post-release mortality. Data suggest that species-specific differences exist wherein winter skate are likely the most resilient (45.4% mortality) to capture and handling, little skate have a moderate capacity to deal with these pressures (62.7% mortality), and barndoor skate appear to be the most susceptible (99.9% mortality). In addition, little and winter skate mortality rates were both influenced by tow duration, while fishing depth and temperature gradient had an effect on little skate and air exposure on winter skate.

Evaluating the condition and discard mortality of winter skate, Leucoraja ocellata, following capture and handling in the Atlantic monkfish (Lophius americanus) sink gillnet fishery

Abstract Discard mortality (DM) estimates are vitally important to fisheries management. Commercial fishery DM estimates, for example, can help calculate total fishing mortality and biologically acceptable catch limits. The winter skate, Leucoraja ocellata, is the only species within the US portion of the western North Atlantic that is targeted in the skate wing fishery. However, due primarily to fishery regulations, this species is also routinely discarded at sea when daily quotas are reached. While already examined in mobile fishing gears (i.e. bottom otter trawls), DM has not been addressed for the western North Atlantic skate species captured in the sink gillnet fishery, and a conservative 50% DM is used across the species of the skate complex in the management processes. Given this data gap, we estimate the DM of winter skate in sink gillnets under standard commercial fishing conditions. The study methods employed address the issue of cryptic mortality (drop out of dead skates from the gear and depredation prior to retrieval), which can bias DM estimates low for bycatch in passive gear. The degree of injury sustained during capture and handling was found to be a statistically significant predictor of mortality. Overall mean DM rates reached an asymptote within 170 h of captivity, with estimates of 11% and 17% for females and males, respectively. Based on these results, the species appears more resilient to sink gillnet capture and release than previously thought. However, cryptic fishing mortality appeared to be high for extended soak times and likely represents an important focus of future research and management actions.

Biosynthesis of 1α-hydroxycorticosterone in the winter skate Leucoraja ocellata: evidence to suggest a novel steroidogenic route.

The present study explores the ability of intracellular bacteria within the renal-inter-renal tissue of the winter skate Leucoraja ocellata to metabolize steroids and contribute to the synthesis of the novel elasmobranch corticosteroid, 1α-hydroxycorticosterone (1α-OH-B). Despite the rarity of C1 hydroxylation noted in the original identification of 1α-OH-B, literature provides evidence for steroid C1 hydroxylation by micro-organisms. Eight ureolytic bacterial isolates were identified in the renal-inter-renal tissue of L. ocellata, the latter being the site of 1α-OH-B synthesis. From incubations of bacterial isolates with known amounts of potential 1α-OH-B precursors, one isolate UM008 of the genus Rhodococcus was seen to metabolize corticosteroids and produce novel products via HPLC analysis. Cations Zn2+ and Fe3+ altered metabolism of certain steroid precursors, suggesting inhibition of Rhodococcus steroid catabolism. Genome sequencing of UM008 identified strong sequence and structural homology to that of Rhodococcus erythropolis PR4. A complete enzymatic pathway for steroid-ring oxidation as documented within other Actinobacteria was identified within the UM008 genome. This study highlights the potential role of Rhodococcus bacteria in steroid metabolism and proposes a novel alternative pathway for 1α-OH-B synthesis, suggesting a unique form of mutualism between intracellular bacteria and their elasmobranch host.

Adaptive phenotypic response to climate enabled by epigenetics in a K-strategy species, the fish Leucoraja ocellata (Rajidae).

The relative importance of genetic versus epigenetic changes in adaptive evolution is a hotly debated topic, with studies showing that some species appear to be able to adapt rapidly without significant genetic change. Epigenetic mechanisms may be particularly important for the evolutionary potential of species with long maturation times and low reproductive potential (‘K-strategists’), particularly when faced with rapidly changing environmental conditions. Here we study the transcriptome of two populations of the winter skate (Leucoraja ocellata), a typical ‘K-strategist’, in Atlantic Canada; an endemic population in the southern Gulf of St Lawrence and a large population on the Scotian Shelf. The endemic population has been able to adapt to a 10°C higher water temperature over short evolutionary time (7000 years), dramatically reducing its body size (by 45%) significantly below the minimum maturation size of Scotian Shelf and other populations of winter skate, as well as exhibiting other adaptations in life history and physiology. We demonstrate that the adaptive response to selection has an epigenetic basis, cataloguing 3653 changes in gene expression that may have enabled this species to rapidly respond to the novel environment. We argue that the epigenetic augmentation of species evolutionary potential (its regulation though gene expression) can enable K-strategists to survive and adapt to different environments, and this mechanism may be particularly important for the persistence of sharks, skates and rays in the light of future climate change.

A generalized model for longitudinal short- and long-term mortality data for commercial fishery discards and recreational fishery catch-and-releases

AbstractConservation concerns and new management policies such as the implementation of ecosystem-based approaches to fisheries management are motivating an increasing need for estimates of mortality associated with commercial fishery discards and released fish from recreational fisheries. Traditional containment studies and emerging techniques using electronic tags on fish released to the wild are producing longitudinal mortality-time data from which discard or release mortalities can be estimated, but where there may also be a need to account analytically for other sources of mortality. In this study, we present theoretical and empirical arguments for a parametric mixture-distribution model for discard mortality data. We show, analytically and using case studies for Atlantic cod (Gadus morhua), American plaice (Hippoglossoides platessoides), and winter skate (Leucoraja ocellata), how this model can easily be generalized to incorporate different characteristics of discard mortality data such as distinct capture, post-release and natural mortalities, and delayed mortality onset. In simulations over a range of conditions, the model provided reliable parameter estimates for cases involving both discard and natural mortality. These results support this modelling approach, indicating that it is well suited for data from studies in which fish are released to their natural environment. The model was found to be less reliable in simulations when there was a delay in discard mortality onset, though such an effect appears only in a minority of existing discard mortality studies. Overall, the model provides a flexible framework in which to analyse discard mortality data and to produce reliable scientific advice on discard mortality rates and possibilities for mitigation.

Fish trophic engineering: Ecological effects of the invasive ascidian Didemnum vexillum (Georges Bank, northwestern Atlantic)

In the northwest Atlantic, concerns for the benthic communities of Georges Bank have evolved following the widespread detection of the invasive ascidian Didemnum vexillum in 2002. One question is whether D. vexillum affects the feeding of fishes, particularly commercially-important species. The major objectives were to examine the diets of five demersal fishes and the benthic epifauna (prey field) across contrasting levels of D. vexillum occurrence from 2004 to 2008 in and around northern Closed Area II of Georges Bank (42.0°N, 67.3°W). The fishes examined were winter skate (Leucoraja ocellata), little skate (Leucoraja erinacea), haddock (Melanogrammus aeglefinus), winter flounder (Pseudopleuronectes americanus), and longhorn sculpin (Myoxocephalus octodecemspinosus). Cumulative fish trophic diversity was often higher at sites where D. vexillum was present as measured by Shannon's H′. Diets were significantly different across levels of D. vexillum for the five fishes, and feeding by haddock was positively correlated with the benthic epifauna within rather than across sites, indicating site-specific feeding. For many fishes, prey that contributed to the diet dissimilarity between sites were benthic epifauna strongly associated with the presence of D. vexillum (Class Polychaeta: Nereis zonata, Harmothoe extenuata, and Lepidonotus squamatus; and Order Decapoda: Cancer irroratus) and absence of D. vexillum (Order Decapoda: Crangon septemspinosa). These feeding alterations are not necessarily negative, as fish diets in this region regularly contain prey positively associated with D. vexillum. However, with the momentum to incorporate habitat science into fish stock assessments, managers should not consider protected fish habitat to be static in the presence of habitat modifiers such as invasive ascidians. From influencing the trophic ecology of demersal fishes to driving substrate homogeneity, long-term monitoring and invasive ascidian management for this continental shelf region is recommended.

Atlantic Rock Crab, unlike American Lobster, Is Important to Ecosystem Functioning in Northumberland Strait

AbstractTo compare the roles of American lobster Homarus americanus and Atlantic rock crab Cancer irroratus in the food web processes of a coastal ecosystem, distribution, abundance, stomach contents, diet overlap, and occurrence in stomachs of predators were described for the two species collected during July–August trawl surveys in Northumberland Strait, southern Gulf of St. Lawrence. Atlantic rock crab was more widely distributed, was more numerous on mud and muddy sand substrates, and occurred in deeper water than American lobster. Atlantic rock crab was the principal prey of American lobster while American lobster was almost never eaten by Atlantic rock crab. American lobster (benthic and pelagic stages) was a trace item in all fish and decapod stomachs examined except in those of the Shorthorn Sculpin Myoxocephalus scorpius and other American lobsters. In contrast, Atlantic rock crab larvae commonly occurred in stomachs of many pelagic fishes while small (<40 mm carapace width) benthic individuals were important prey of demersal fish species such as Winter Skate Leucoraja ocellata, Cunner Tautogolabrus adspersus, Longhorn Sculpin M. octodecemspinosus, and Shorthorn Sculpin. Atlantic rock crab had a broad diet consisting of fish, sevenspine bay shrimp Crangon septemspinosa, Atlantic rock crab (mostly cannibalism), bivalves, and polychaetes. American lobster had a narrow diet consisting mainly of Atlantic rock crab with lesser amounts of American lobster (old carapaces and cannibalism) and sea stars. Diet overlap between the two species was moderate and mainly due to joint consumption of Atlantic rock crab. Thus, Atlantic rock crab play a prominent role in the southern Gulf of St. Lawrence food web (and therefore energy cycling) while American lobster serves mostly as an energy sink whose loss from the ecosystem would mainly affect people dependent upon the fishery.Received January 3, 2014; accepted May 30, 2014

Mercury bioaccumulation in cartilaginous fishes from Southern New England coastal waters: Contamination from a trophic ecology and human health perspective

This study examined total mercury (Hg) concentrations in cartilaginous fishes from Southern New England coastal waters, including smooth dogfish (Mustelus canis), spiny dogfish (Squalus acanthias), little skate (Leucoraja erinacea), and winter skate (Leucoraja ocellata). Total Hg in dogfish and skates were positively related to their respective body size and age, indicating Hg bioaccumulation in muscle tissue. There were also significant inter-species differences in Hg levels (mean ± 1 SD, mg Hg/kg dry weight, ppm): smooth dogfish (3.3 ± 2.1 ppm; n = 54) > spiny dogfish (1.1 ± 0.7 ppm; n = 124) > little skate (0.4 ± 0.3 ppm; n = 173) ∼ winter skate (0.3 ± 0.2 ppm; n = 148). The increased Hg content of smooth dogfish was attributed to its upper trophic level status, determined by stable nitrogen (δ15N) isotope analysis (mean δ15N = 13.2 ± 0.7‰), and the consumption of high Hg prey, most notably cancer crabs (0.10 ppm). Spiny dogfish had depleted δ15N signatures (11.6 ± 0.8‰), yet demonstrated a moderate level of contamination by foraging on pelagic prey with a range of Hg concentrations, e.g., in order of dietary importance, butterfish (Hg = 0.06 ppm), longfin squid (0.17 ppm), and scup (0.11 ppm). Skates were low trophic level consumers (δ15N = 11.9–12.0‰) and fed mainly on amphipods, small decapods, and polychaetes with low Hg concentrations (0.05–0.09 ppm). Intra-specific Hg concentrations were directly related to δ15N and carbon (δ13C) isotope signatures, suggesting that Hg biomagnifies across successive trophic levels and foraging in the benthic trophic pathway increases Hg exposure. From a human health perspective, 87% of smooth dogfish, 32% of spiny dogfish, and <2% of skates had Hg concentrations exceeding the US Environmental Protection Agency threshold level (0.3 ppm wet weight). These results indicate that frequent consumption of smooth dogfish and spiny dogfish may adversely affect human health, whereas skates present minimal risk.

Maturity, size at age and predator–prey relationships of winter skate Leucoraja ocellata in the southern Gulf of St Lawrence: potentially an undescribed endemic facing extirpation

The goals of this study were to document the size and age structure, size at maturity, ovarian fecundity and diet of the endangered population of winter skate Leucoraja ocellata that resides in the southern Gulf of St Lawrence (SGSL). The maximum size observed for SGSL L. ocellata was 68 cm total length (LT ) but >99% of animals caught were <60 cm LT . Fifty per cent of male and female L. ocellata were fully mature at 40 and 42 cm LT , respectively, age c. 5 years. The oldest individual caught was age 11 years, but 98% of the 561 individuals examined were ≤age 8 years, indicating a short reproductive life span. Ovarian fecundity was low; no more than 29 ova >10 mm diameter were ever observed. At 40 cm LT , the diet changed from one dominated by shrimp Crangon septemspinosa and gammarid amphipods to one dominated by fishes (mainly sand lance Ammodytes spp. and rainbow smelt Osmerus mordax) and Atlantic rock crab Cancer irroratus. Sufficient differences were observed between SGSL L. ocellata and other populations in their size-at-maturity pattern and maximum size to propose the taxonomic re-evaluation of the population.

Abundance, distribution and habitat characteristics of winter skate Leucoraja ocellata in the southern Gulf of St Lawrence: a population on the brink of extirpation?

Abundance indices of southern Gulf Leucoraja ocellata residing in Northumberland Strait, Canada, declined from 2001 to 2005, but increased during 2006 to 2008. Catch rates in 2009 were the lowest in the time series. The size of the area occupied in Northumberland Strait varied with changes in the abundance indices. Leucoraja ocellata were primarily caught in the north-west half of the strait over sandy to gravelly seabed; a near total absence was documented from the eastern half despite suitable habitat and a previously established presence documented during research surveys conducted before 2000. In Northumberland Strait, L. ocellata occupied shallow (50% cumulative occurrence = 12 m) and warm (50% cumulative occurrence = 16·5° C) coastal waters and were not present in the cold (<1° C, >35 m) intermediate layer. Abundance and distribution data highlight the need for continuing studies to monitor the status of this population, which is classified as endangered by the IUCN and Committee on the Status of Endangered Wildlife in Canada.

Moving beyond the current paradigm in marine population connectivity: are adults the missing link?

AbstractDuring the past century, the field of fisheries oceanography has dominated the study of population connectivity in marine environments. The influence of physical and biological processes and their relationship to transport and retention of early life history stages has been central in providing insight into population structuring and connectivity. However, the focus on dispersive early life history stages has meant that the role of adults has received less attention and is not fully understood or appreciated. We argue that adults play a vital role in population connectivity for a wide range of marine taxa and hypothesize that adult‐mediated population connectivity commonly results in a diverse array of population structuring. Two case‐studies on winter skate, Leucoraja ocellata, and winter flounder, Pseudopleuronectes americanus, are presented to illustrate the role adults play in marine connectivity at both broad and fine scales, respectively. Indeed, if adults are important for population connectivity, we argue that the role of larval processes is conditional on adult choice and only management and research pursuits that integrate the full life cycle of species will capture the full dynamics of metapopulation connectivity. Failure to include the roles of adults can lead to misinterpretation of the causes and consequences of changes in ecosystem structure and fisheries productivity.

Contrasting decadal trends in mortality between large and small individuals in skate populations in Atlantic Canada

Mature thorny (Amblyraja radiata), winter (Leucoraja ocellata), and smooth (Malacoraja senta) skates have declined to very low abundance in the southern Gulf of St. Lawrence (SGSL) and on the eastern Scotian Shelf (ESS). We used stage-structured state-space models to examine decadal patterns in mortality rates in these skates. Mortality at early life stages (embryos in egg cases, hatchlings, and (or) small juveniles) appeared to decrease between the 1970s and the 2000s. In contrast, estimated mortality rates increased for larger individuals over this period. Although potentially confounded in models with effects of any changes in juvenile growth, the estimated increases in mortality could not instead be attributed solely to changes in growth. Increases in the mortality of large individuals appeared to reflect increases in natural mortality, possibly due to predation by grey seals. Increases in natural mortality were not evident for skates on the neighbouring western Scotian Shelf, where grey seal abundance has remained lower. Even in the absence of fishing, recovery of skates is unlikely under current ecosystem conditions in the SGSL and on the ESS.

Short-term post-release mortality of skates (family Rajidae) discarded in a western North Atlantic commercial otter trawl fishery

Due to market and regulatory factors, Rajidae skates are routinely discarded by commercial otter trawlers in the western North Atlantic. Accounting for post-release mortality is therefore essential to total fishing mortality estimates, stock status and management of this group of fishes. However, despite a presumed species-specific range in tolerance, few studies have investigated the short-term post-release mortality among skates indigenous to the western North Atlantic following capture by mobile fishing gears, and never in the Gulf of Maine. This study addresses this shortfall for the prohibited thorny skate, Amblyraja radiate and smooth skate, Malacoraja senta, and the targeted winter skate, Leucoraja ocellata, and little skate, Leucoraja erinacea. Of 1288 skates evaluated, negligible immediate mortality was observed at the time of capture, even in relation to the largest catches and/or most prolonged tows. However, injury frequency was moderate, with highest levels in the smooth (60%) and thorny (52%) skates. Aside from the smooth skate (59%), 72h mortality rates were low overall (19% across all species when accounting tow durations indicative of the fishery), with the winter skate (8%) exhibiting the lowest levels. Logistic regression modeling revealed tow duration as the most universal predictor of condition and 72h mortality, while catch biomass, sex, temperature changes, and animal size also held influence in certain species. Although in general the studied species appear more resilient to trawl capture and handling than previously estimated, interspecific differences must be accounted for when managing this group.