Francois Poisson

A review of capture and post‐release mortality of elasmobranchs

There is a need to better understand the survivorship of discarded fishes, both for commercial stocks and species of conservation concern. Within European waters, the landing obligations that are currently being phased in as part of the European Unions reformed common fisheries policy means that an increasing number of fish stocks, with certain exceptions, should not be discarded unless it can be demonstrated that there is a high probability of survival. This study reviews the various approaches that have been used to examine the discard survival of elasmobranchs, both in terms of at-vessel mortality (AVM) and post-release mortality (PRM), with relevant findings summarized for both the main types of fishing gear used and by taxonomic group. Discard survival varies with a range of biological attributes (species, size, sex and mode of gill ventilation) as well as the range of factors associated with capture (e.g. gear type, soak time, catch mass and composition, handling practices and the degree of exposure to air and any associated change in ambient temperature). In general, demersal species with buccal-pump ventilation have a higher survival than obligate ram ventilators. Several studies have indicated that females may have a higher survival than males. Certain taxa (including hammerhead sharks Sphyrna spp. and thresher sharks Alopias spp.) may be particularly prone to higher rates of mortality when caught.

Large-scale genetic panmixia in the blue shark ( Prionace glauca ): a single worldwide population, or a genetic lag-time effect of the “grey zone” of differentiation?

The blue shark Prionace glauca, among the most common and widely studied pelagic sharks, is a top predator, exhibiting the widest distribution range. However, little is known about its population structure and spatial dynamics. With an estimated removal of 10–20 million individuals per year by fisheries, the species is classified as “Near Threatened” by International Union for Conservation of Nature. We lack the knowledge to forecast the long‐term consequences of such a huge removal on this top predator itself and on its trophic network. The genetic analysis of more than 200 samples collected at broad scale (from Mediterranean Sea, North Atlantic and Pacific Oceans) using mtDNA and nine microsatellite markers allowed to detect signatures of genetic bottlenecks but a nearly complete genetic homogeneity across the entire studied range. This apparent panmixia could be explained by a genetic lag‐time effect illustrated by simulations of demographic changes that were not detectable through standard genetic analysis before a long transitional phase here introduced as the “population grey zone.” The results presented here can thus encompass distinct explanatory scenarios spanning from a single demographic population to several independent populations. This limitation prevents the genetic‐based delineation of stocks and thus the ability to anticipate the consequences of severe depletions at all scales. More information is required for the conservation of population(s) and management of stocks, which may be provided by large‐scale sampling not only of individuals worldwide, but also of loci genomewide.

Catch, Bycatch of Sharks, and Incidental Catch of Sea Turtles in the Reunion-Based Longline Swordfish Fishery (Southwest Indian Ocean) Between 1997 and 2000

Data from voluntary logbooks (5,884 longline sets) collected between 1997 and 2000 were analyzed to assess the potential impact of the Reunion-based longline swordfish fishery (South West Indian Ocean) on sharks and on sea turtle populations. Blue shark (Prionace glauca) represented between 75% and 88% of the total catches of sharks studied. Bycatch discarding varied with species, ranging from low discards (2.6%) for mako shark (Isurus sp.) to high discards for blue shark (86.5%). Estimation of the total catch of sharks (retained and discarded individuals) represented in weight between 7% and 9% of the total catch of the major species caught by the fishery. Of concern is the decline of blue shark CPUE (Catch Per Unit of Effort) from 2.2 to 1.03 sharks per 1,000 hooks between 1998 and 2000. Of the 22,974 hook-timers deployed during cruises onboard commercial, 49% of the blue shark (Prionace glauca) and 41.2% of the oceanic whitetip shark (Carcharhinus longimanus) were retrieved alive. It was assumed that among the 15,250 blue sharks caught during this period, at least 7,099, released alive, could have survived.