Haldorson et al.: Spring abundance patterns of marine fish larvae 



37 



Figure 1 



Auke Bay study area in southeast Alaska, and location of the Auke Bay Monitor (ABM I 

 station. 



Materials and methods 



Fish larvae were collected in Auke 

 Bay at a station designated ABM 

 (Fig. 1) from mid-March or early 

 April through mid-June, 1986-89. 

 The ABM station was selected be- 

 cause it had been used in previ- 

 ous studies (summarized in Coyle 

 & Shirley 1990). Samples were 

 collected on the same day each 

 week between 0800 and 1300, 

 with the exception on the second 

 week of April 1986 (Fig. 4). Each 

 week five replicate samples were 

 collected with a 1 nr Tucker trawl 

 constructed of 505 |x mesh and 

 fitted with a digital flowmeter in 

 the middle of the net opening. 

 Each replicate was collected at the 

 ABM station by towing the net in 

 a double-oblique trajectory to a 

 depth of 30-35 m. The vessel 



systems. The water column is isothermal until April, 

 when surface warming and increased freshwater run- 

 off contribute to formation of a pycnocline (Bruce et 

 al. 1977, Ziemann et al. 1991). In the 4 years of this 

 study water temperature prior to stratification var- 

 ied from 3 to 5 C, was colder in 1986 and 1989, and 

 warmer in 1988 (Fig. 2; data from Ziemann et al. 

 1990). Stratification, indicated by diverging tempera- 

 tures at 5 and 20m, began in April (Fig. 2; data 

 from Ziemann et al. 1990). 



Auke Bay exhibits a typical subarctic annual pro- 

 duction cycle (Williamson 1978, Ziemann et al. 1991). 

 The spring phytoplankton bloom began in early April, 

 1986-89 (Fig. 3; data from Ziemann et al. 1990), in 

 response to several consecutive days of relatively 

 high light levels (Ziemann et al. 1991). Chlorophyll 

 biomass peaked in late April or early May each year, 

 with a subsequent decline resulting from nutrient 

 limitation (Ziemann et al. 1991). The herbivorous 

 copepod maximum began 2-4 weeks after the spring 

 phytoplankton bloom (Fig. 3; data from Coyle & Paul 

 1990), with Pseudocalanus spp. copepods dominant 

 in every year (Coyle et al. 1990). Copepod nauplii in 

 the size-ranges consumed by larval fishes were typi- 

 cally in low density prior to the herbivorous copepod 

 maximum and reached maximum density in May, 

 although there was considerable interannual varia- 

 tion in nauplii density during the period of peak 

 abundance (Paul et al. 1991). 



