Abstract
Sperm depletion in males can occur when polygynous species are intensively exploited under a male-biased management strategy. In fisheries involving crabs species, the effects of this type of management on the reproductive potential is far from being understood. This study tests whether male-biased management of the principal Chilean crab fishery is able to affect the potential capacity of Metacarcinus edwardsii males to transfer sperm to females. Five localities in southern Chile, recording contrasting crab fishery landing, were selected to assess the potential of sperm depletion triggered by fishery. Seasonally, male crabs from each locality were obtained. Dry weight and histological condition of vasa deferentia and the Vaso-Somatic Index (VSI) were determined in order to use them as proxies for sperm depletion and male reproductive condition. A manipulative experiment was performed in the laboratory to estimate vasa deferentia weight and VSI from just-mated males in order to obtain a reference point for the potential effects of the fishery on sperm reserves. Sperm storage capacity is significantly affected by fisheries; during the mating season vasa deferentia from localities with low fishery intensity were heavier than those from high intensity fisheries, and these differences were even more evident in large males. Histological section showed that this disparity in vasa deferentia weight was explained principally by differences in the quantity of spermatophores rather than other seminal material. VSI was always higher in males from localities with low fishery intensity. Males from localities with high fishery intensity showed little capacity to recover sperm reserves and the VSI of these males remained below the values of the just-mated males. Detriment in the capacity of males to transfer sperm is the first step to sperm limitation in an exploited population, thus detection of sperm depletion can be an alert to introduce changes in the current management of crabs.
Highlights
Fishery activity is a source of selective mortality for wild populations
Deviation from a 1:1 sex ratio for physiologically mature crabs was only significantly detected in Calbuco (3.1 males: 1 female, χ2 = 10.6, p = 0.03, n = 5), while the other locality with low fishery intensity, Los Molinos, had similar tendency (Fig. 4) but deviation was not significant (p>0.05)
All localities in the high fishery intensity category (Ancud, Dalcahue and Quellon) recorded non-biased sex ratio for mature crabs (Fig. 4)
Summary
Fishery activity is a source of selective mortality for wild populations. This is more evident when management measures target a specific size and sex, changing the structure of the population under exploitation. This is the case in most crab fisheries, which typically select for large, dominant males. This selective pressure leads to changes in sex ratios in favor of females [1] and possibly sperm limitation [2,3,4]. When dominant males are scarce due to fishery pressure, remaining males mate repeatedly in order satisfy the sperm demand of females [5] and potential for sperm depletion appears
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