Stock Identity of Elasmobranchs in the Northeast Atlantic in Relation to Assessment and Management

Abstract

Fish stock assessment requires that those parts of a species' range can be identified in which the effects of exploitation in particular fisheries are apparent. Thus, fisheries are monitored and regulated in management units largely determined by the distribution of fisheries, areas which may or may not coincide with biological self-perpetuating units. There are several methods of identifying fish stocks, including the interpretation of distribution and relative abundance, tagging studies, genetic analyses and spatial variation in morphometrics, life-history parameters, parasites and contaminants. The natural variability and difficulty in measuring life history and morphological characteristics at the population level suggest that these are not good indicators of stock separation for elasmobranchs. Information on population demography does, however, reveal the relationships between fisheries and resource, and indicates where sampling programmes for the collection of stock assessment data should take place. Tagging studies, in particular, can indicate a stock's main range and migrations, if linked with a good description of the fisheries that are involved with its exploitation. The environmental parameters associated with the distribution of a species are also a useful guide to the likely boundaries of movement. In this paper, we discuss the utility of these approaches to delineate areas to be used for stock assessments of elasmobranchs, and show how biological and fishery information has been used to describe the stock identity for six case study species in the Northeast Atlantic.

Long-term change in a meso-predator community in response to prolonged and heterogeneous human impact

“…Devoid of pelagic larval stages, elasmobranchs disperse relying on intrinsic mobility and available ecological corridors48. The species that were dominant in the upper Adriatic can undertake transoceanic and long seasonal migrations494850, and have home ranges much larger than the northern Adriatic. Hence they may be little affected by localized stressors because they are able to replenish from areas even outside the Adriatic, possibly using relatively sheltered gateways in Croatian coastal waters.…”

“…Hence they may be little affected by localized stressors because they are able to replenish from areas even outside the Adriatic, possibly using relatively sheltered gateways in Croatian coastal waters. Conversely, elasmobranchs with limited mobility such as catsharks and other skates (home ranges <50 km)4749 persisted in the less-exploited eastern Adriatic and almost completely disappeared from the heavily exploited Italian waters.…”

Comparative rates of growth of the Port Jackson shark throughout its southern Australian range

“…Yet, differences among estimates of individual growth parameters, particularly k and L N , because these two parameters are highly correlated, may be confounded by the sampling limitations of the different studies (Francis and Francis 1992;Pawson and Ellis 2005). The present study, like Tovar-Á vila et al (2009b), employed a sampling methodology that precluded the capture of the largest size cohorts of the respective populations.…”

“…growth, recruitment, and natural and fishing mortality (Begg et al 1999b). For elasmobranchs, spatial differences in one or more life-history parameter provide a common means for investigating population structuring, because of the relative ease of obtaining estimates of these parameters (Begg et al 1999b;Pawson and Ellis 2005). Several species of elasmobranchs have been shown to display latitudinal and/or geographic differences in rates of reproduction and growth (e.g.…”

Winter residency and site association in the Critically Endangered North East Atlantic spurdog Squalus acanthias

“…A more recent study has disputed this suggestion (Vince 1991), indicating a single large migratory stock of spurdog within the UK, with a high national dispersal of juvenile and maturing animals. On this basis, the NE Atlantic population of spurdog is assessed as one stock (Pawson & Ellis 2005). Females appear to aggregate in the eastern Celtic Sea in order to parturate during winter and spring (Pawson 1995).…”