Hyperiid amphipods from the Gulf of Ulloa and offshore region, Baja California: Intermittent use of the coastal shelf

Hyperiid amphipod species from the Gulf of Ulloa, Baja California, and the adjacent region (from shelf break to 200 km offshore) were analyzed in order to evaluate diversity and abundances in this productive area that supports small-scale commercial fisheries such as barred sand bass (), California spiny lobster (), abalones, clams, and others. Strong coastal upwelling events were observed during summer seasons of the period 2002-2008 between Punta Eugenia and Punta Abreojos. The upwelling plumes at Punta Abreojos are projected southward in slope waters bordering the coastal shelf of the Gulf of Ulloa, contributing to the separation of coastal and oceanic regions, and explain differences in amphipod diversity and abundances. In the offshore region, the most abundant species were , and , similar to previous findings in northern regions of Baja California and southern California. However, their abundances were lower (between 10 and 30 individuals/1000 m), only reaching 20-50% of abundance levels reported off northern Baja California. In the coastal shelf of the Gulf of Ulloa, amphipods were virtually absent during 2002, 2003 and 2006. However, during 2004 and 2005, abundances of increased (54 and 20 ind/1000 m, respectively). Moreover, during 2007, abundances of , and increased considerably (261, 39, 31, 68, 416 ind/1000 m, respectively), indicating occasional utilization of the coastal shelf by pelagic amphipods. Gelatinous organisms paralleled changes in hyperiid populations and were particularly abundant in 2007 in the coastal shelf. Significant correlations of 17 amphipod species with gelatinous taxa, which are often used as host organisms by hyperiid amphipods, suggest that those organisms enhanced hyperiid abundance and promoted the progression onto the coastal shelf during some years of the 2002-2008 period.