Understanding and managing fish populations: keeping the toolbox fit for purpose.

Wild fish populations are currently experiencing unprecedented pressures, which are projected to intensify in the coming decades. Developing a thorough understanding of the influences of both biotic and abiotic factors on fish populations is a salient issue in contemporary fish conservation and management. During the 50th Anniversary Symposium of The Fisheries Society of the British Isles at the University of Exeter, UK, in July 2017, scientists from diverse research backgrounds gathered to discuss key topics under the broad umbrella of 'Understanding Fish Populations'. Below, the output of one such discussion group is detailed, focusing on tools used to investigate natural fish populations. Five main groups of approaches were identified: tagging and telemetry; molecular tools; survey tools; statistical and modelling tools; tissue analyses. The appraisal covered current challenges and potential solutions for each of these topics. In addition, three key themes were identified as applicable across all tool-based applications. These included data management, public engagement, and fisheries policy and governance. The continued innovation of tools and capacity to integrate interdisciplinary approaches into the future assessment and management of fish populations is highlighted as an important focus for the next 50 years of fisheries research.

Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes.

Biological transfer of nutrients and materials between linked ecosystems influences global carbon budgets and ecosystem structure and function. Identifying the organisms or functional groups that are responsible for nutrient transfer, and quantifying their influence on ecosystem structure and carbon capture is an essential step for informed management of ecosystems in physically distant, but ecologically linked areas. Here, we combine natural abundance stable isotope tracers and survey data to show that mid-water and bentho-pelagic-feeding demersal fishes play an important role in the ocean carbon cycle, bypassing the detrital particle flux and transferring carbon to deep long-term storage. Global peaks in biomass and diversity of fishes at mid-slope depths are explained by competitive release of the demersal fish predators of mid-water organisms, which in turn support benthic fish production. Over 50% of the biomass of the demersal fish community at depths between 500 and 1800 m is supported by biological rather than detrital nutrient flux processes, and we estimate that bentho-pelagic fishes from the UK-Irish continental slope capture and store a volume of carbon equivalent to over 1 million tonnes of CO2 every year.

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.

Identifying migrations in marine fishes through stable-isotope analysis.

The isotopic composition of many elements varies across both land and ocean surfaces in a predictable fashion. These stable-isotope ratios are transferred into animal tissues, potentially providing a powerful natural geospatial tag. To date, most studies using stable isotopes as geolocators in marine settings have focussed on mammals and seabirds conducting large ocean-basin scale migrations. An increasing understanding of isotopic variation in the marine environment, and improved sampling and analytical techniques, however, means that stable isotopes now hold genuine promise as a natural geolocation tag in marine fishes. Here, the theoretical background underpinning the use of stable isotopes of C, N and O in otolith, scale and muscle tissues as geolocation tools in the marine environment is reviewed, and examples of their applications are provided.

Can otolith elemental chemistry retrospectively track migrations in fully marine fishes?

Otolith microchemistry can provide valuable information about stock structure and mixing patterns when the magnitude of environmental differences among areas is greater than the cumulative influence of any vital effects. Here, the current understanding of the underlying mechanisms governing element incorporation into the otolith is reviewed. Hard and soft acid and base (HSAB) theory is employed to explore the differences in chemical behaviours, distributions and affinities between elements. Hard acid cations (e.g. Mg(2+) , Li(+) and Ba(2+) ) tend to be less physiologically influenced and accepted more readily into the otolith crystal lattice but are relatively homogeneous in seawater. Soft acid cations (e.g. Zn(2+) and Cu(2+) ) on the other hand, exhibit more varied distributions in seawater, but are more likely to be bound to blood proteins and less available for uptake into the otolith. The factors influencing the geographical distribution of elements in the sea, and their incorporation into the otoliths of marine fishes are reviewed. Particular emphasis is placed on examining physiological processes, including gonad development, on the uptake of elements commonly used in population studies, notably Sr. Finally, case studies are presented that either directly or indirectly compare population structuring or movements inferred by otolith elemental fingerprints with the patterns indicated by additional, alternative proxies. The main obstacle currently limiting the application of otolith elemental microchemistry to infer movements of marine fishes appears to lie in the largely homogeneous distribution of those elements most reliably measured in the otolith. Evolving technologies will improve the discriminatory power of otolith chemistry by allowing measurement of spatially explicit, low level elements; however, for the time being, the combination of otolith minor and trace element fingerprints with alternative proxies and stable isotopic ratios can greatly extend the scope of migration studies. Among the otolith elements that routinely occur above instrument detection limits, Ba, Mn and Li were deemed the most likely to prove reliable geographic markers in marine species.

Effect of vitamin D deficiency during pregnancy on offspring bone structure, composition and quality in later life.

During foetal development, calcium requirements are met as a consequence of maternal adaptations independent of vitamin D status. In contrast, after birth, dependency on vitamin D appears necessary for calcium metabolism and skeletal health. We used a rodent model (Sprague-Dawley rats), to determine if maternal vitamin D deficiency during pregnancy had a deleterious effect on bone structure at birth. Vitamin D deplete females were maintained under deplete conditions until birth of the pups, whereupon all dams were fed a vitamin D replete diet. Offspring were harvested at birth, and 140 days of age. Bones were analyzed using micro-computed tomography and strength tested to study differences in bone structure, density and strength and subjected to elemental analysis using plasma mass spectrometry to determine strontium, barium and calcium contents. Offspring from deplete mothers displayed altered trabecular parameters in the femur at birth and 140 days of age. In addition, at 140 days of age there was evidence of premature mineralization of the secondary ossification centre of the femoral head. Elemental analysis showed increased strontium uptake in the femur of the developmentally vitamin D-deficient offspring. Vitamin D depletion during development in the offspring may have a long-lasting effect, despite repletion of vitamin D from birth. This may have consequences for human health given the low vitamin D levels seen during pregnancy and current lifestyle of sun avoidance due to the risk of skin cancer.

Towards the development of a fossil bone geochemical standard: an inter-laboratory study.

Ten international laboratories participated in an inter-laboratory comparison of a fossil bone composite with the objective of producing a matrix and structure-matched reference material for studies of the bio-mineralization of ancient fossil bone. We report the major and trace element compositions of the fossil bone composite, using in-situ method as well as various wet chemical digestion techniques. For major element concentrations, the intra-laboratory analytical precision (%RSD(r)) ranges from 7 to 18%, with higher percentages for Ti and K. The %RSD(r) are smaller than the inter-laboratory analytical precision (%RSD(R); <15-30%). Trace element concentrations vary by approximately 5 orders of magnitude (0.1 mg kg(-1) for Th to 10,000 mg kg(-1) for Ba). The intra-laboratory analytical precision %RSD(r) varies between 8 and 45%. The reproducibility values (%RSD(R)) range from 13 to <50%, although extreme value >100% was found for the high field strength elements (Hf, Th, Zr, Nb). The rare earth element (REE) concentrations, which vary over 3 orders of magnitude, have %RSD(r) and %RSD(R) values at 8-15% and 20-32%, respectively. However, the REE patterns (which are very important for paleo-environmental, taphonomic and paleo-oceanographic analyses) are much more consistent. These data suggest that the complex and unpredictable nature of the mineralogical and chemical composition of fossil bone makes it difficult to set-up and calibrate analytical instruments using conventional standards, and may result in non-spectral matrix effects. We propose an analytical protocol that can be employed in future inter-laboratory studies to produce a certified fossil bone geochemical standard.