Interactions of the fishery of the spider crab Maja squinado with mating, reproductive biology and migrations

Abstract

[Abstract] In this paper different aspects of the fishery and life history of the spider crab Maja squinado in southern Galicia (NW Spain) are analyzed to evaluate the potential effects of the fishery on the sperm limitation of the reproductive effort (egg production) of the population. Juveniles of the spider crab inhabit shallow waters, where they carry out a terminal moult in August-September, attaining sexual maturity when they are 2+ years old. A short time after the terminal moult (October-November), adults migrate to deeper waters (up to 100 m), where mating occurs (January-February). Field and laboratory data show that multiple matings and sperm storage in female seminal receptacles occur, indicating that females are able to fertilize multiple broods during the annual breeding cycle using stored sperm. The spider crab is the target of a tangle-net fishery, characterized by a very high fishing effort similar for both sexes. The fishing season is from November-December until May-June and is mostly dependent on migrating animals. Data from catch composition (percentage of recent recruits at the beginning of the season), recaptures from the fishery of females tagged with ultrasonic transmitters and electronic archival tags, and CPUE trends over the course of the fishing season (Leslie analyses of stock depletion) indicate that more than 90% of postpubertal (primiparous) adults are caught during the fishing season. The fishery is almost exclusively dependent on the autumn recruitment of the annual cohort of primiparous adults. Most of the catches are made in autumn and early winter, before or during the mating season (for 4 fishing seasons an average of 45 and 66% of the catches are made before January and February respectively). The volume of sperm stored in the seminal receptacles and the percentage of females with sperm is lower for females caught in the field during or immediately after the mating season than for females kept in laboratory with a high density of males, and decreases throughout the annual breeding cycle. These two facts may be brought about the low density of both sexes in the mating grounds due to fishery exploitation. We hypothesized that, although sperm limitation probably does occur, the main factor in the severe fishery induced limitation of the reproductive effort of the Galician spider crab populations, as opposed to other crab fisheries targeting only males, is the mortality imposed on females before or during breeding. More information about mating habitat, seasonality and behaviour would be needed for an accurate evaluation of the potential effects of the fishery (and of different management strategies) on sperm limitation.