Larval fish habitats and hydrography in the Biosphere Reserve of the Upper Gulf of California (June 2008)

Abstract

The Upper Gulf of California (UGC) is a Biosphere Reserve that despite its extreme environmental conditions (macrotidal inverse estuary) houses a high fish species richness. An intensive sampling of fish larvae and hydrography was carried out during June 2008 in the UGC. From 56 zooplankton sampling stations with a maximum of three sampling strata (each 5 m deep, from 0 to 15 m), a total of 29,505 fish larvae were collected, included in 99 taxa and 32 families. The Bray–Curtis Index defined three main larval fish habitats that varied in composition. (i) The “Mixed” larval habitat was mostly defined in the vertically mixed western sector of the UGC; the coastal pelagic Anchoa spp. presented high abundance in this habitat, associated with demersal species such as Gobulus crescentalis and Scianidae type 1. The lowest diversity and abundance, and the highest salinity, temperature and chlorophyll distinguished this larval habitat. (ii) The “Front” habitat was located mostly on the physical–chemical frontal zone between the UGC and the Northern Gulf; it had the highest specific richness and larval abundance. The dominant species were the coastal pelagics Anchoa spp. and Opisthonema sp. 1; the latter was almost limited to the north by the frontal zone. (iii) The “Shelf” habitat, found over the shelf off the mainland, was the deepest and less salty, and was also dominated by Opisthonema sp. 1, but included epipelagic species such as Scombridae (e.g., Scomber japonicus, Auxis spp., Scomberomerus sierra), probably from the adjacent deeper zone. These larval fish habitats had well-defined limits that coincided with marked environmental gradients, with the lowest larval diversity in the saltiest environment; this suggests that the human-induced shift to hypersaline conditions may have reduced the preferred larval habitat for some species. The habitats most likely change with the seasons, with implications for the management of the reserve.