Summary1. Sharks and rays are vulnerable to fisheries exploitation because of late maturation and low fecundity, highlighting the need for effective conservation strategies. Area closures have been proposed as an appropriate management option for thornback rays in the southern North Sea, where they appear to form local subpopulations between which there is limited mixing. 2. To gain a fishery-independent estimation of stock distribution, 197 thornback rays Raja clavata tagged with electronic data storage tags (DST) were released in the Thames Estuary in 1999 and 2000. 3. The tidal location method was used to estimate the positions of individual fish between time of release and recapture. The fishery-independent seasonal stock distributions were integrated with landings data and a simple model was developed to estimate monthly fishing effort per International Council for the Exploration of the Sea (ICES) rectangle by different fleets and gear types. 4. The potential impacts of closed area management, in terms of reducing thornback ray mortality in the southern North Sea, were calculated. Spatial closures were applied either (i) as permanent closures of individual ICES rectangles or (ii) seasonally, at the level of the Thames Estuary. Catch reductions were calculated allowing for the redistribution of fishing effort. 5.The results confirmed the importance of the Thames Estuary for thornback rays. However, 77% of rays moved outside the estuary over winter, with seasonal migration into the Thames to spawn between March and August. 6. The effects of closures varied between areas and gear types. Permanent closures of individual ICES rectangles were less effective at reducing fishing mortality on rays than a spring or summer closure of the Thames Estuary as a whole, which would have a major impact on the commercially more valuable sole fishery. 7. Synthesis and applications . The results presented illustrate the potential impacts of a range of closure scenarios, prior to their implementation, as a basis for advice on sustainable exploitation of thornback rays. These models could be further refined by additional studies of juvenile behaviour and of other ray subpopulations in the southern North Sea.

BackgroundSexual selection theory predicts that females, being the limiting sex, invest less in courtship signals than males. However, when chemical signals are involved it is often the female that initiates mating by producing stimuli that inform about sex and/or receptivity. This apparent contradiction has been discussed in the literature as 'the female pheromone fallacy'. Because the release of chemical stimuli may not have evolved to elicit the male's courtship response, whether these female stimuli represent signals remains an open question. Using techniques to visualise and block release of urine, we studied the role of urine signals during fighting and mating interactions of crayfish (Pacifastacus leniusculus). Test individuals were blindfolded to exclude visual disturbance from dye release and artificial urine introduction.ResultsStaged female-male pairings during the reproductive season often resulted in male mating attempts. Blocking female urine release in such pairings prevented any male courtship behaviour. Artificial introduction of female urine re-established male mating attempts. Urine visualisation showed that female urine release coincides with aggressive behaviours but not with female submissive behaviour in reproductive interactions as well as in intersexual and intrasexual fights. In reproductive interactions, females predominately released urine during precopulatory aggression; males subsequently released significantly less urine during mating than in fights.ConclusionsUrine-blocking experiments demonstrate that female urine contains sex-specific components that elicit male mating behaviour. The coincidence of chemical signalling and aggressive behaviour in both females and males suggests that urine release has evolved as an aggressive signal in both sexes of crayfish. By limiting urine release to aggressive behaviours in reproductive interactions females challenge their potential mating partners at the same time as they trigger a sexual response. These double messages should favour stronger males that are able to overcome the resistance of the female. We conclude that the difference between the sexes in disclosing urine-borne information reflects their conflicting interests in reproduction. Males discontinue aggressive urine signalling in order to increase their chances of mating. Females resume urine signalling in connection with aggressive behaviour, potentially repelling low quality or sexually inactive males while favouring reproduction with high quality males.

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