Auke Bay Laboratory (ABL)

Groundfish Assessment Program

Daily Predictability of Prey Available to Free-Ranging Steller Sea Lions in Southeast Alaska

Pacific herring are a major prey species for Steller sea lions in Southeast Alaska. During winter, herring in this region aggregate in dense, layered schools associated with the seafloor. If sea lions can easily find these aggregations, the net benefits from foraging trips are maximized. Predictability of prey aggregations, both in time and location, decreases search time and improves foraging profitability.

Monthly prey surveys, conducted in a previous study between June 2001 and May 2004, indicated that over-wintering herring aggregations occur consistently each year near Benjamin Island, an important sea lion haulout in Lynn Canal north of Juneau. During winter 2005, we conducted new prey surveys in this area at weekly and daily intervals to examine predictability of herring schools over shorter time scales than those examined in the 2001–04 study. Results from this fine-scale study will provide further insight into 1) the energetic benefit that sea lions derive from herring, and 2) the balance between predation avoidance by herring and the costs associated with searching by sea lions.

Figure 1.  Southeast Alaska acoustic survey area.

Replicate acoustic surveys were conducted from October 2004 to April 2005 to evaluate the daily predictability of available prey to Steller sea lions. The survey area is located in Favorite Channel and north Stephens Passage (Fig. 1) south of the Benjamin Island haulout, and encompasses a nearly continuous submarine gully interrupted only by glacial outwash from Eagle River. The survey area appears to form important winter habitat for herring in Southeast Alaska. Five surveys of 70 nautical miles (nmi) length each were conducted per month.

In addition, gill-net sampling and midwater trawl sampling were used to collect biological samples and identify echo sign from the acoustic surveys. Gill-net sampling was conducted 2 days per month, and midwater trawl sampling was conducted for 3 days each quarter. Acoustic surveys were conducted from the fishing vessel (FV) Sea View, gill-net sampling from the FV Irish Eyes, and midwater trawl sampling from the research vessel (RV) Medeia, each of which were chartered for this study.

In previous study years, midwater trawling was used exclusively to collect biological samples. Gill-net sampling methods were tested in 2004–05 because the charter vessel cost for gill-net sampling is about one-third of the cost for midwater trawl sampling. Gill-net sampling methods were developed and tested by ABL scientists J.J. Vollenweider and Dave Csepp and were effective for sampling near-bottom forage species, including herring and eulachon.

Figure 2.  An acoustic echogram of two large, dense Pacific herring schools in their fall schooling behavior near Benjamin Island, Alaska. Figure 3.  An acoustic echogram of Pacific herring in typical, dense winter layering behavior in the Amalga Harbor area near Juneau, Alaska.

Herring behavior in 2004–05 was similar to behavior in previous years. Few herring were found during October, and large, dense schools of herring appeared near Benjamin Island in November (Fig. 2). In December, one-half of the herring moved south into Favorite Channel (Amalga Harbor) and by January and February, the majority of the herring formed dense layers there (Fig. 3). The southward movement continued in March, when herring were concentrated near Portland Island. The winter layer began breaking up in March, forming a thinner layer and starting to form separate, scattered schools.

Adult eulachon were mixed with juvenile walleye pollock in midwater trawl sampling and gill-net sampling. In addition to the replicate surveys, an acoustic survey was conducted in Berners Bay north of Benjamin Island in January that targeted larval eulachon and capelin. The acoustic survey showed two distinct weak layers throughout the bay, one layer at 50 m depth and another at 100 m. Midwater trawling of these layers in December caught larval capelin at 50 m depth and larval eulachon at 100 m.

By David Csepp and Mike Sigler

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AFSC Quarterly Research Reports Jan-Mar 2005