Project Title: Temporal stability of fatty acids used to discriminate Pacific herring in Alaska

Abstract
Statement of Problem
Full Proposal

Principal Investigator: Ted Otis, Alaska Dept. of Fish and Game, Homer, AK   
Co-Principal Investigator: Ron Heintz, National Marine Fisheries Service-Auke Bay Lab, Juneau, AK

Funding Agency: Exxon Valdez Oil Spill Trustee Council
http://www.evostc.state.ak.us/

Abstract:

This project follows up on a promising pilot study that demonstrated the ability to discriminate Alaska herring stocks at relatively fine spatial scales (> 100 km) based on the fatty acid composition of their heart tissue.  The investigators propose to assess the temporal stability and biological variability of stock discrimination criteria derived from fatty acid analysis of herring cardiac tissues.   Samples will be collected during the spring and fall/winter of 2005 and 2006 from putative herring stocks from Sitka, PWS, Kamishak, Kodiak, Dutch Harbor, Togiak, and Kuskokwim Bay in the Bering Sea.  Results should allow managers to better define ecologically significant stock boundaries, which would likely affect how commercially exploited herring populations are assessed and managed.  Results will be published in a peer-reviewed report and may lead to revision of fishery management plans for affected areas. 

Keywords: Pacific herring, stock identification, fatty acid analysis, Gulf of Alaska

Statement of Problem:

Despite decades of study and over a hundred years of commercial exploitation targeting Pacific herring (Clupea pallasi), considerable uncertainty continues to exist regarding: 1) the scale at which population structure exists within large geographic areas and, 2) the degree to which herring return to natal areas to spawn.  These fundamental life history traits are directly relevant to how exploited herring stocks should be assessed and managed (Hourston 1982; Wheeler and Winters 1984; Hay and McCarter 1997; McQuinn 1997).  State fishery managers require a tool that can identify ecologically significant population structuring among adjacent spawning aggregations that are exploited during spring sac-roe herring fisheries. They also require a mixed stock analysis tool that allows them to investigate whether winter herring fisheries (e.g., food/bait fisheries) target only the local spawning stock or a mixture of nearby stocks that aggregate during winter.  The ability to manage stocks discretely is a principal component of sustainable fisheries management- one that requires the ability to accurately apportion the catch from mixed stock fisheries. 

Researchers have attempted to use many different techniques to distinguish among herring stocks, including: scale pattern analysis (Rowell 1981), tagging studies (Hourston 1982), morphometrics and meristics (Schweigert 1990), microsatellite DNA (O’Connell et al. 1998), and otolith microchemistry (Otis and Heintz 2003).  However, most techniques have proven to be unreliable at fine spatial scales.  For example, O’Connell et al. (1998) found that herring from Prince William Sound (PWS) and the Bering Sea were genetically divergent, but they were unable to find similar divergence among stocks sampled within the north Gulf of Alaska.  The difficulty encountered with genetic markers is likely due to the relatively high stray rates exhibited by herring (e.g., Tester 1949; Cushing and Burd 1957; Hourston 1982; Wheeler and Winters 1984).  Very little gene flow between populations is necessary to compromise the ability of allozyme markers to discriminate among putative stocks (Smith and Jamieson 1986; Bembo et al. 1996; Waples 1998).  In particular, Waples (1998) observed that “because the amount of migration necessary to obscure most genetic evidence of stock structure (only a handful of individuals per generation) is generally inconsequential as a force for rebuilding depleted populations on a time scale of interest to humans, there is no guarantee that genetic methods alone will provide sufficient precision for key management decisions involving marine species”.  Thus, herring managers have continued to seek a tool that allows them to identify population structure within and among their respective management areas. 

In the absence of more definitive tools, many fishery managers have traditionally used spawning timing and location as proxies to roughly define herring stock structure.  The logical assumption is, the greater the temporal and spatial separation between spawning aggregates, the greater the likelihood that they are discrete stocks. However, problems can arise when mixing of putative stocks occurs across jurisdictional boundaries.  Anecdotal observers have reported examples in which the abundance of one presumptive spawning stock “crashes” while an adjacent area’s presumptive stock simultaneously increases by a commensurate amount. Such observations of “spawner relocation” highlight the behavioral complexity of herring (Overholtz 2002; Hay and McKinnell 2002; Huse et al. 2002) and raises questions regarding stock discreteness and population “sub-units” (Stephenson 1999).

Recently, a new method of stock identification was applied successfully to discriminate known herring stocks and reveal differences among putative stocks at relatively fine spatial scales (> 100 km). The method discriminates stocks using differences in the fatty acid composition of cardiac tissue (Otis and Heintz 2003).  This method has been tested for other fish species (e.g. Grahl-Nielsen and Mjaavatten 1992, Castell et al. 1995, Pickova et al. 1997, Joensen et. al. 2000) but requires further testing before it can be applied to herring.  To date, these tests indicate that the fatty acid composition of cardiac tissues are the least influenced by environmental factors (Viga and Grahl-Nielsen 1990), is sensitive at discriminating stocks over small geographic scales (Grahl-Nielsen and Mjaavatten 1992) and has a genetic basis (Joensen et al. 2000).

Whether or not detection of discernable differences in arbitrarily selected variables constitutes ecologically significant, distinct populations is open to debate (Waples 1998).  That debate has particular relevance to this proposal since many studies have shown that the fatty acid compositions of some tissues and lipid classes are highly sensitive to changes in diet and the environment (e.g., Hazel 1984, Henderson and Tocher 1987, Cordier 2002).  Therefore, demonstrating that the variation in heart tissue fatty acid composition observed between stocks exceeds that imposed by the environment on a given stock will be a key element in the development of this method (Begg et al. 1999).  We are proposing to target heart tissues because heart phospholipids are less subject to environmental influences than other tissues or lipid classes (Grahl-Nielsen and Ulvund 1990, Czesny et al. 2000, McKenzie 2001).  Several studies have shown that dietary impacts on fatty acid composition are minimized in heart lipids. Viga and Grahl-Nielsen  (1990) cultured groups of Atlantic salmon from the same stock for eight months on prescribed diets and found the fatty acid composition of salmon hearts was independent of diet. Grisdale-Helland et al. (2002) found significant differences in the heart phospholipids of Atlantic salmon fed different diets for approximately three months. However, they identified much greater differences in the composition of heart triacylglycerols. Similarly, studies reviewed by McKenzie (2001) reveal the tendency for heart fatty acid composition to respond to diet but at much lower magnitude than muscle or liver. These data indicate that examination of heart fatty acids should minimize the apparent variation imposed on populations due to diet, ration, and temperature (Grisdale-Helland et al. 2001; Kiessling et al. 2001; Jobling et al. 2002).  

Three recent laboratory studies reported evidence of genetic control over some fatty acid concentrations, thus supporting the premise that they can be effective stock identifiers. Joensen et al. (2000) found significant differences in the fatty acid profiles of heart tissue extracted from representatives of two cod stocks that had been reared for 44 months under identical diets and environments. Peng et al. (2003) compared the fatty acid compositions of anadromous and landlocked Atlantic salmon (Salmo salar) fry, fed identical diets throughout a 44-day feeding trial, and reported significant differences in their phospholipids. In a companion study, Rollin et al. (2003) concluded that differences in the fatty acid composition of different strains of Atlantic salmon resulted from variation in the rates of desaturation and elongation of linolenic and linoleic acids. This suggests that differences in the activities of enzymes that regulate phospholipid composition might explain the stock differences identified here and in other species examined in field studies (Grahl-Nielsen and Ulvund 1990, Grahl-Nielsen and Mjaavatten 1992).
 
The concept of genetic control over the composition of heart fatty acids is bolstered by studies demonstrating relationships between cardiac function and fatty acid composition. Bell et al. (1993) reported heart lesions in Atlantic salmon fed diets with high levels of n-6 fatty acids after the fish had been stressed. Agnisola et al. (1996) reported reduced heart rate and cardiac power output in the hearts of sturgeon fed diets high in n-3 fatty acids relative to those fed diets high in n-6 fatty acids. These data demonstrate an influence of heart fatty acid composition on individual fitness, thereby providing a basis for differences among reproductively isolated aggregates.  Alternatively, interactions between phospholipid composition, eicosanoid production and cardiac function have rarely been described for fish (Stenslokken et al. 2002) despite their frequently described impacts on mammalian health (Das 2001). These data may account for the conclusion that C22:6n3 in fish heart phospholipids is not strongly influenced by diet (Thomassen and Røsjø 1989, Caballero et al. 2002, Grisdale-Helland 2002), and in fact may be under strong genetic control (Peng et al. 2003).