Growth and development of Pseudocalanus spp. in the northern Gulf of Alaska

Pseudocalanus are the numerically dominant calanoid species in coastal subarctic Pacific waters. We examined their juvenile growth rates, and explored their relationships to temperature, chlorophyll a and body size for Pseudocalanus spp. from 2002 to 2004 in the northern Gulf of Alaska. Generally, the monthly mean growth rates increased from 0.049 +/- 0.007(SE) day(-1) in March to 0.095 +/- 0.016 day(-1) in August, declining slightly to 0.074 +/- 0.009 day(-1) in October. Typically, growth rates at most stations were around 0.05 day(-1), with no consistent or significant pattern between stations. After standardization to 5 and 10 degrees C, the mean growth rates were 0.045 +/- 0.002 day(-1) and 0.075 +/- 0.004 day(-1) respectively, with growth rate decreasing with increasing development stage. Unlike other local calanoid copepod species, Pseudocalanus species tend to be more temperature-dependent than food-dependent, with composite statistical models describing at most 30% of the observed variability in growth rate. Interestingly, development time was comparable to other co-occurring calanoid copepods; however, growth rates of Pseudocalanus spp. were considerably lower. We demonstrate this with a new multi-species model that describes the growth rates of other egg-scattering copepods in this ecosystem, but to which Pseudocalanus does not fit. Thus, the egg-carrying Pseudocalanus species appear to employ a life history strategy optimized for slow growth at low chlorophyll that keeps individuals relatively small, and may therefore reduce visual predation upon them.