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Use of Fatty Acids for Discriminating Forage Fish Stocks Over Fine Spatial Scales
Stock assessments for forage fish depend on the ability to identify stocks, but stock structure has proven difficult to discriminate over fine spatial scales (i.e., small differences in locations of fish stocks). A variety of approaches have been employed including parasite markers, meristic comparisons, scale pattern analysis, electrophoresis, and DNA analysis, but none of these have proven to be totally effective.
This problem is particularly vexing in the northern Gulf of Alaska (NGA), where mixed herring stocks are commercially fished without an understanding of the underlying population structure and where spatially discrete populations are sometimes managed as a single unit. Our objective was to develop a method that could discriminate herring stocks over fine spatial scales. To meet this objective, the Auke Bay Laboratory (ABL), in conjunction with the Alaska Department of Fish and Game (ADF&G), compared the use of otolith microchemistry and fatty acid analysis of heart lipids as tools for discriminating NGA herring stocks.
We sampled spawning herring from a diversity of sites, some adjacent in geographic location and others separated distinctly. This allowed us to examine the utility of these methods for discriminating stocks known to differ genetically, and then apply the methods to stocks that have proven difficult to discriminate.
Otoliths and hearts were sampled from spawning populations in spring 2001. Approximately 30 female herring from Sitka Sound, Togiak Bay, Kodiak, Kamishak Bay in Cook Inlet, Montague Island, and Fairmont Bay in Prince William Sound were sampled. Initially, the samples from Kodiak, Kamishak, Montague, and Fairmont were combined into a single group identified as NGA. These were combined with the samples from Sitka and Togiak to determine if the methods worked as well as microsatellite DNA (a method typically employed for identifying fish stocks). Then the analysis was repeated with just the NGA stocks.
The elemental composition of herring otoliths, as determined by
electron microprobe, was useful for discriminating stocks on the basis
of the early marine experience of the herring, but not on the basis of
geographic location (Fig. 1). The most important elements in the
discriminant functions were Na and Cl, and these could be used to
discriminate groups with low NaCl in their otolith foci from those with
relatively high NaCl in their otolith foci. Thus, stocks spawning in
locations open to the Gulf such as Kodiak, Montague, and Sitka were
generally indistinguishable. However, they were readily distinguished
from Fairmont, Kamishak, and Togiak. If the NGA samples were assigned to
one of these two identities based on the NaCl content of their otoliths,
then unknown samples could be correctly identified more than 90% of the
In contrast, the fatty acid compositions of herring heart lipids were useful for discriminating herring stocks at broad and fine spatial scales. The fatty acid compositions of hearts from NGA, Sitka, and Togiak herring differed, and the identity of samples of unknown origin could be correctly determined more than 95% of the time. Fatty acids could also be used to discriminate the stocks over fine spatial scales (Fig. 2). The compositions of heart lipids of the NGA stocks were found to differ, and the overall error rate determined from cross-validation was less than 10%. Error rates for individual stocks ranged between 8%-12% and errors were generally made by assigning individuals to the stock of nearest geographic proximity.
These data demonstrate the potential of fatty acids for discriminating herring stocks over relatively fine spatial scales. However, the data included in the current models are derived from samples collected concurrently. It is not known if these models could correctly identify individuals sampled at other times of year. We are currently processing samples of herring that were collected several months after the samples described here. These analyses will provide a further test of the utility of fatty acid analysis for discriminating forage fish stocks over fine spatial and temporal scales.
The data also indicate potential uses of otolith micro-chemistries for examining the early life history of herring stocks. The groups identified by elemental composition of otoliths coincided with those known to spawn in waters directly exposed to the Gulf versus those known to spawn in locations with more freshwater influence. This could prove particularly useful for identifying stocks when only hard body parts are available. For example, it may be possible to determine if pinnipeds are foraging on stocks from either the inside or the outside waters of southeastern Alaska by examining the elemental composition of otoliths recovered in scats.
By Ron Heintz.
A leading hypothesis regarding the decline of the Alaskan Steller sea lion (SSL) stocks is the so-called Junk Food Hypothesis, suggesting that poor quality and low diversity of sea lion prey are prevalent factors contributing to the decline. Scientists from ABL have determined the nutritional qualities of various SSL prey species detected in scat analyses using proximate analysis to quantify available energy. In addition, fatty acid compositions have been determined for a number of prey species for use in stock separation and identification.
One SSL prey species under examination is eulachon (Thaleichthys pacificus), due to its high overall lipid content and subsequent energetic value in the diet. Each spring, hundreds of sea lions gather and forage on eulachon that aggregate in recurrent locations before beginning their spawning runs.
These eulachon “feasts” are some of the earliest major food sources available in spring and are thought to be important energy-rich meals prior to the sea lions’ migration to rookeries to pup and reproduce. Adequate levels of fat intake and ingestion of essential fatty acids (EFAs) are important for healthy pup development.
Eulachon were collected and analyzed each season over the course of 16 months and in pre- and post-spawning states from Lynn Canal and Frederick Sound in Southeast Alaska. Proximate analysis indicated that the mean lipid content of the eulachon varied seasonally from 11% to 19%. While these values are significantly higher than most SSL prey species, eulachon lipid contents were near minimal values during the time of the spawning aggregation.
Presumably, this reduction in lipid composition is due to the energetic cost of egg production and migration to the spawning site. Given their high abundance and low energy expenditure by predators, however, the eulachon run remains a valuable energy resource for the sea lions.
Prior to breeding, sea lion cows require energy stores for their fasting periods and lactation. In addition, high levels of EFAs, particularly those of the omega-3 and omega-6 families, are important for healthy pup development. Fatty acid compositions were determined for eulachon, indicating that they contain very low levels of omega-3 and omega-6 EFAs. Intriguingly, significant seasonal variations were observed for the omega-3 EFAs, while only minor changes were seen for most of the omega-6 EFAs. Eulachon appear to be a particularly poor source of the omega-3 EFAs docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), with mean values of 3.4% for DHA and 3.1% for EPA of the total fatty acid composition.
Other important SSL prey items such as herring, walleye pollock, Pacific cod, and sand lance range from 13% to 22% DHA and 8% to 13% EPA of the total fatty acid composition (based on data from Iverson et al., Mar. Ecol. Prog. Ser. 151: 255-71, 1997). Furthermore, eulachon possess a mean overall omega-3/omega-6 ratio of only 4.3, while herring, pollock, Pacific cod, and sand lance have mean omega-3/omega-6 ratios ranging from 9 to 25.
Though eulachon are an energy-rich food source and are readily available at a time just before the breeding season begins, they appear to be one of the poorest sources of EFAs in the known diet of SSL. Insufficient levels of EFA intake in mammals have been linked to various types of birth defects. Thus, eulachon may not be the best high quality SSL prey item, as previously thought, suggesting the need for another source of EFAs available to the SSL before the beginning of the breeding season.
By Lawrence Schaufler and Johanna Vollenweider.
Auke Bay Lab