Seasonal growth, molt, and egg production of the euphausiid Nyctiphanes simplex (Crustacea: Euphausiacea) in the Gulf of California

Abstract

Seasonal distribution of biomass and somatic growth, molt, and egg production rates of Nyctiphanes simplex were estimated in the Gulf of California (November 2005, January 2007and July 2007) and associated with 12 environmental variables to define the conditions in which the higher biomass production rates occur. Daily growth rates, estimated from shipboard incubations, indicated that N. simplex did not grow or decreased in size in all the seasons, with higher proportions of animals in these 2 growth categories in July (21 and 52%, respectively) than in January (7 and 43%) and November (35 and 12%). Thus, the proportion of individuals that grew was higher in November (53%) and January (50%) than in July (27%). Mean juvenile and adult intermolt period (IMP) based on direct measurements was 5 d in January (range: 3 to 7 d), 3.8 d in July (2 to 8 d) and 4.4 d (3 to 7 d) for September to October 2010 (used as proxy for November, since not enough IMP data were available for that month). Calculations based on the inverse molting rate method estimated higher mean IMP (6.7 d; range 4 to 60 d). N. simplex mean (±SD) juvenile and adult daily total biomass production rate was 0.16 ± 0.13 mg DW m−3 d−1 (DW = dry weight) and mean annual integrated production was 71 ± 58 mg DW m−3 yr−1. The greatest contribution to N. simplex biomass production was through somatic growth (46%), followed by molts (32%) and eggs (22%). N. simplex had high biomass and biomass production rates mostly in regions with <100 m seafloor depth, in the northeast Gulf during November, northwest during January, and along the central east coast of the Gulf during July, associated with upwelling conditions that geographically change with winter anticyclonic and summer cyclonic surface circulation. Thus, numerous predators that energetically depend on this prey must seasonally change their distribution pattern to detect the spatially variable centers of N. simplex biomass production in the Gulf of California.