Samuel Shephard

Spatial Heterogeneity in Fishing Creates de facto Refugia for Endangered Celtic Sea Elasmobranchs

The life history characteristics of some elasmobranchs make them particularly vulnerable to fishing mortality; about a third of all species are listed by the IUCN as Threatened or Near Threatened. Marine Protected Areas (MPAs) have been suggested as a tool for conservation of elasmobranchs, but they are likely to be effective only if such populations respond to fishing impacts at spatial-scales corresponding to MPA size. Using the example of the Celtic Sea, we modelled elasmobranch biomass (kg h−1) in fisheries-independent survey hauls as a function of environmental variables and ‘local’ (within 20 km radius) fishing effort (h y−1) recorded from Vessel Monitoring Systems data. Model selection using AIC suggested strongest support for linear mixed effects models in which the variables (i) fishing effort, (ii) geographic location and (iii) demersal fish assemblage had approximately equal importance in explaining elasmobranch biomass. In the eastern Celtic Sea, sampling sites that occurred in the lowest 10% of the observed fishing effort range recorded 10 species of elasmobranch including the critically endangered Dipturus spp. The most intensely fished 10% of sites had only three elasmobranch species, with two IUCN listed as Least Concern. Our results suggest that stable spatial heterogeneity in fishing effort creates de facto refugia for elasmobranchs in the Celtic Sea. However, changes in the present fisheries management regime could impair the refuge effect by changing fishers behaviour and displacing effort into these areas.

Fishing for MSY: using “pretty good yield” ranges without impairing recruitment

28 Pretty Good Yield (PGY) is a sustainable fish yield corresponding to obtaining no less than a specified large 29 percentage of the Maximum Sustainable Yield (MSY). We investigated 19 European fish stocks to test the 30 hypothesis that 95% PGY yield range is inherently precautionary with respect to impairing recruitment. An FMSY 31 range was calculated for each stock as the range of fishing mortalities (F) that lead to an average catch of at 32 least 95% of MSY in the long term simulations. Further, a precautionary reference point for each stock (FP.05) 33 was defined as the F resulting in a 5% probability of the spawning stock biomass falling below an agreed 34 biomass limit below which recruitment is impaired (Blim) in long‐term simulations. For the majority of the stocks 35 analysed, the upper bound of the FMSY range exceeded the estimated FP.05. However, larger fish species had 36 higher precautionary limits to fishing mortality, and species with larger asymptotic length were less likely to 37

Thermal, trophic and metabolic life histories of inaccessible fishes revealed from stable‐isotope analyses: a case study using orange roughy Hoplostethus atlanticus

A time-resolved record of inhabited water depth, metabolic rate and trophic behaviour of the orange roughy Hoplostethus atlanticus was recovered from combined stable-isotope analyses of otolith and muscle tissue. The results demonstrate that H. atlanticus from the north-east Atlantic Ocean have a complex life history with three distinct depth-stratified life stages. Early juvenile H. atlanticus occupy relatively shallow habitats, juvenile H. atlanticus show a deep-demersal phase, rising at sexual maturity, and adult H. atlanticus exploit increasingly deep habitats with increasing age. At all sampled sizes, H. atlanticus muscle tissues have an isotopic composition suggesting a benthic rather than benthopelagic or pelagic diet. Isotopic measures of relative metabolic rate provide an insight into energy partitioning throughout ontogeny. Hoplostethus atlanticus have relatively low metabolic rates compared to coexisting deep-water benthic fishes, consistent with their unusually high longevity. Surprisingly, lifetime fastest growth rates are achieved during juvenile stages when otolith isotopes imply deep-water residency and relatively low metabolic rates. Fast growth may be sustained during a period of high efficiency associated with reduced metabolic costs of prey capture or predator evasion. The stable-isotope approach can be applied to any teleost and provides a rapid, cost-effective technique for studying deep-water fish communities.

Parallel decadal variability of inferred water temperatures for Northern and Southern Hemisphere intermediate water masses

We use a novel proxy (growth rates of long-lived deep water fish, orange roughy) to reconstruct inferred water temperatures of intermediate water masses in both Northern and Southern Hemispheres since the mid-1800s. The data are consistent with instrumental records showing long-term warming in the Northern Hemisphere but also indicate decadal variability of intermediate depth temperatures that is coherent across the two hemispheres. This variability correlates with the dominant subpolar annular mode in each hemisphere and implies a bihemispheric oceanic response to external forcing that influences the properties of intermediate depth water masses.