Environmental influence on the phenology of Mustelus schmitti and seasonal availability for small-scale fisheries

Abstract

This study investigates how environmental factors (temperature, chlorophyll, turbidity, wind, freshwater discharge) and time (month) influence life history stage availability of M. schmitti in a reproductive aggregation area, San Antonio Cape. Based on the landing of a gill net fishery, this research was conducted during 18 months covering two austral reproductive periods. A total of 13,633 individuals (TL 38–95 cm) were examined, and both sexes showed a multimodal length distribution with three main size assemblages (TL < 50 cm, 50–70 cm, TL > 72 cm) showing monthly occurrence differences, ocurring all size assemblages, two or just one. Both sexes showed higher densities from September to November (spring), albeit with strong differences between both spring periods, and lower densities during summer and winter. The MRT analysis showed that main size/sex- classes segregation was associated to shallow inshore vs. deeper waters (16.32 %), followed by turbidity (8.01 %, Kd490) within inshore waters and SST/freshwater discharge (~13.96 ◦ C/~21,369 m3s-1, equal improve, 5.05 %) at the deeper waters; and the subsequent to SST values. The intra-annual environmental variation associated to size/sex-classes suggests that the occurrence of seasonal life cycle events could be explained by the species’s size/sex-classes sensitivity to changes in the spatial extent of environmental conditions (i.e., temperature and freshwater discharge). The timing of these events appears to be driven more by environmental shifts than by other factors in this coastal habitat. Small-scale fishery landings offered a useful context to infer how environmental factors influence the timing of availability of different shark life stages.