Comparing Atlantic and Mediterranean populations of the velvet belly lanternshark, Etmopterus spinax, with comments on the efficiency of density-dependent compensatory mechanisms

Abstract

Etmopterus spinax is a small-sized deep-water lantern shark that occurs in the Eastern Atlantic and the Mediterranean. Differences in depth distribution, densities, size at maturity and fecundity were compared between a population that has suffered high levels of fishing mortality during the last decades (Southern Portugal in the northeast Atlantic) and a population where low fishing pressure below 500 m occurs at present or has occurred in the last decades (Northern Alboran Sea in the western Mediterranean). The density of this species, as derived by experimental bottom trawl survey, off the coast of Southern Portugal, is substantially lower than in the Northern Alboran Sea throughout the entire depth range. The Atlantic population is maturing at smaller sizes than the Mediterranean population and has a lower mean fecundity. Specifically, sizes at maturity for Southern Portugal and the Northern Alboran Sea were, respectively, 25.39 and 28.31 cm TL for males and 30.86 and 34.18 cm TL for females, while mean fecundities for Southern Portugal and the Northern Alboran Sea were, respectively, 9.94 and 11.06 oocytes per mature female. This work demonstrated the possible presence of density-dependent mechanisms in the Southern Portuguese population of E. spinax that has lowered the size at maturity as a possible result of excessive fishing mortality. However, given that this is an aplacentary viviparous shark, where fecundity is dependent on female size, this compensatory mechanism seems to have a limited efficiency.

Keywords:

• Density-dependent mechanism
• elasmobranch
• fecundity
• fishing effects
• length at maturity

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

R. Coelho received a PhD grant from FCT, the Foundation for Science and Technology (Ref. SFRH/BD/10357/2002), co-funded by POCI 2010 and FSE. The authors are grateful to all fishermen who collaborated in collecting specimens for this study. Special thanks go to I. Diogo and F. Diogo, skippers of the longliner ‘Branca de Sagres’ and H. Cavaco, skipper of the bottom trawler ‘Gamba’. The authors wish to thank I. Figueiredo and P.B. Machado (INIAP – IPIMAR) for providing the samples collected during this institute's annual demersal trawl surveys.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark