Aging fish otoliths recovered from Pacific harbor seal fecal samples

Fishery Bulletin, Oct, 2006 by Susan D. Riemer, Robert Mikus

Seals and sea lions are opportunistic predators that feed on a variety of fish and cephalopods, including some commercially and recreationally important species. Concerns over their interactions with commercial and sport fishing operations, and other human activities, has a long history in the Pacific Northwest (Everitt and Beach, 1982). The perceived increase in such interactions led, in part, to the U.S. Congress amending the Marine Mammal Protection Act (MMPA) in 1994. Chief among the amendments was a call for research to determine 1) whether California sea lions (Zalophus californianus) and Pacific harbor seals (Phoca vitulina) were affecting the recovery of listed or depleted salmonids, and 2) what broader impacts they may have on the coastal ecosystems of Oregon, Washington, and California (NMFS, 1997). In this note, we begin to address the latter question. Specifically, we describe a novel application of otolith aging techniques that can be used to increase the understanding of pinniped foraging ecology and, thus, their potential impact on the fishery resources of coastal ecosystems.

The Oregon Department of Fish and Wildlife's (ODFW) Marine Mammal Research Program has studied harbor seal foraging habits since the mid-1980s, primarily through the collection and analysis of scat (fecal) samples (Riemer and Brown, 1997; Riemer et al. (1); Wright et al. (2)). Our results, as well as those from other pinniped food habit studies in Oregon (Graybill, 1981; Brown and Mate, 1983; Roffe and Mate, 1984; Orr et al., 2004), indicate that these animals consume a large number of diverse prey species. Concurrent research in Oregon indicates that Pacific harbor seals have increased significantly following protection under the MMPA; Brown et al. (2005) estimated that the 2002 statewide population total was 10,087 individuals. As noted above, these types of increases in pinniped abundance have led to more frequent interactions with coastal fish resources and hence an increased interest in the composition and abundance of prey in their diet.

Fortunately, new techniques have recently been developed to analyze pinniped diets. For example, rather than using traditional methods that rely strictly on otoliths (fish ear bones) for identifying prey, many researchers now try to identify all skeletal structures recovered from scats, which provide a more complete picture of pinniped diets (Olesiuk et al., 1990; Cottrell et al., 1996; Riemer and Brown, 1997; Browne et al., 2002). In addition, researchers are beginning to use molecular genetic methods to provide greater resolution in determining diet composition (Purcell et al., 2004; Deagle et al., 2005; Kvitrude et al., 2005). In this note, we add to this growing list of techniques by describing a novel use of fish otolith aging techniques to further our understanding of pinniped diets.

Fisheries scientists have aged fish otoliths to aid in the management of commercial and recreational fisheries (Love et al., 2002). Marine mammal scientists, on the other hand, have relied upon the identification of otoliths recovered from scats to identify the prey of pinnipeds (Brown and Mate, 1983; Beach et al. (3); Harvey, 1989; Pierce and Boyle, 1991). However, the age of prey has rarely been considered when describing seal and sea lion diets. We believe that estimates of the age of prey will result in a more comprehensive picture of pinniped diets and help to augment stock assessments that use age-specific models. We report our application of these techniques to Dover sole (Microstomus pacificus) otoliths recovered from Pacific harbor seal scat samples collected in an Oregon estuary. Dover sole were selected as a case study because they are a common prey of harbor seals in Oregon (Riemer and Brown, 1997) and their otoliths have been aged successfully in previous studies (Hagerman, 1952; Brodziak and Mikus, 2000).

Materials and methods

We conducted our study during the spring and summer of 1996, and year-round from 1997 through 2002, in the Alsea Estuary located near Waldport, Oregon (44[degrees]26'N, 124[degrees]3'W). The local harbor seal population in this area consisted of approximately 600 animals throughout the study period. Scat samples from this population were obtained during low tides by approaching haul-out areas on foot or by boat, and slowly moving the animals into the water. Samples were placed in individually labeled plastic bags and frozen. The number of scat samples collected during each trip varied depending on the number and location of animals hauled out, and on weather and ocean conditions.

Scats were thawed and partially dissolved in water, then rinsed through a series of nested sieves (2 mm, 1 mm, 0.71 mm). All prey hard parts (e.g., otoliths, bones) recovered were dried and placed in individually labeled jars. Prey species were identified from all prey hard parts recovered from each sample. Dried hard parts were examined under a dissecting microscope and identified by using a comparative collection of fishes from the northeast Pacific Ocean and Oregon estuaries. Otoliths and diagnostic bones were identified, counted, and the side (left or right) was noted to estimate a minimum number of individuals (MNI) represented in each sample by following the procedures described by Lance et al. (4)