SEXUAL SEGREGATION IN MARINE PREDATORS: PATTERMS, MECHANISMS AND CONSERVATION IMPLICATIONS

Sexual segregation is widespread in the animal kingdom, yet the underlying causes remain controversial. Investigative studies to date have largely concentrated on sexually size-dimorphic terrestrial species such as ungulates. However, these may not be the most appropriate models as intersexual size differences are likely to confer significant differences in attributes such as predation risk, nutritional requirements and energy budgets, all of which may influence the habitat selection of the sexes.

This research uses an integrative approach of field-based tracking and complementary, hypothesis-driven laboratory studies to investigate the function of sexual segregation in the lesser spotted catshark, Scyliorhinus canicula, a model species which is sexually monomorphic with respect to body size.

The behaviour of free-ranging catsharks has been studied in detail in Lough Hyne, Ireland, since 1996 using acoustic and archival tracking of individuals over a range of spatio-temporal scales. This work has revealed sex differences in both vertical and horizontal space use. Females refuge in shallow water (3 – 5 m) caves for up to 75 % of the time, but undertake nocturnal foraging excursions to depth (≤ 50 m). In contrast, males exhibit saltatory behaviour in comparatively deep water (18-20 m) by day, whereas nocturnally either range throughout the water column or occupy shallow-water habitats.

Hypotheses informed by observations of wild behaviour have been examined in the laboratory to gain a better understanding of potential mechanisms. Four hypotheses that could account for this sex difference in habitat use have been systematically investigated; namely, the acitvity budget, predation risk, thermal niche-fecundity, and social factors hypotheses.

We also use fishery catch data to map sexual segregation patterns at the large scale. Current investigations include studies of shortfin mako and blue sharks (see Biol. Lett., 2009).

We have also produced a review on sexual segregation in marine vertebrates (Adv. Mar. Biol., 2008).

People involved: Dr Viki Wearmouth, Dr Emily Southall, Nuno Queiroz, Professor David Sims (MBA), Dr David Morrit (Royal Holloway, University of London).

Funding: NERC, Fisheries Society of the British Isles.