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UI WLF 448 - Variation of Chinook Salmon Fecundity

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1 Variation of Chinook salmon fecundity between the main stem of the Naknek River and a tributary stream, Big Creek in southwestern Alaska for submission to the North American Journal of Fisheries Management Matthew L. Albert Department of Fish and Wildlife Resources University of Idaho Moscow, Idaho 83844-1136 (208) 301-3658 [email protected] Abstract I estimated fecundities for two stocks of Chinook salmon Oncorhynchus tshawytscha from the same drainage in southwestern Alaska using a length based linear regression. Female Chinook salmon from the main stem of the Naknek River had a mean fecundity of 9,852 eggs per fish. Big Creek, a tributary stream, had a mean fecundity of 9,060 eggs per female. The observed difference in fecundities was found to be significant (p < 0.001). I was able to attribute this difference primarily to a difference between the size class structures of the two stocks. I also eliminated other alternative hypothesis as to why these stocks had different fecundities. I then speculated that there are selective pressures selecting for smaller spawners in Big Creek mainly due to shallow water depth. Further research is needed before this hypothesis can be validated or management recommendations can be made. Accurate fecundity estimates are important for understanding dynamics of fish populations, predicting trends in population abundance, and estimating spawning-stock biomass (Eldridge and Jarvis 1995). Reproductive potential influences the ability of a species to respond to abiotic and/or biotic stresses. Knowledge of the fecundity of a stock is needed to quantify the effects of external stresses such as fishing, on the reproductive potential of the species (Nitschke and Mather 2001). Fecundity is also of theoretical interest because the energy invested in egg production cannot be used for other functions like growth, escaping predators, or foraging (Healey and Heard 1984). In Pacific salmon Oncorhynchus spp., fecundity varies widely between species, with in populations and even more so between populations (Healy and Heard 1984).3 Understanding fecundity in salmonids is of interest because they produce a relatively small number of large eggs compared to other fish species. This suggests a demonstrable relationship between the reproductive potential of the spawning stock and the numbers of young surviving (Rounsefell 1957). For salmonids, there is a positive relationship between body length of a mature female and fecundity (Fleming and Gross 1990). The Naknek River Drainage supports anadromous runs of all five species of Pacific salmon and numerous resident fish species. Chinook salmon O. tshawytscha return to the Naknek River from late May through August. The Naknek River is one of the most accessible rivers in southwest Alaska. It also supports one of the largest Chinook salmon returns in the region. Consequently, the river supports a large sport fishery for Chinook salmon. Recent creel surveys have shown effort to be at 15,512 angler-days which constitutes 16 percent of the effort in southwest Alaska (Dunaway et al. 2000). Fisheries managers should take fecundity of the specific stocks that they are monitoring into account when making decisions about harvest and escapement goals, especially in a mixed stock fishery with stocks that have significantly different fecundities. The goal of this study is to determine if the main stem of the Naknek River and Big Creek Chinook Salmon have different fecundities and if so what changes to the management of the fishery should occur to keep both stocks at maximum sustained yield. Study Area The Naknek River drainage is in the Bristol Bay region of Southwest Alaska (Figure 1). The Naknek River originates from Naknek Lake and flows for approximately4 40 km before entering Bristol Bay. It is a large clear water river with a maximum width of approximately 100 m and strong flow (Schwanke 2002). Big Creek is an important Chinook salmon producing tributary of the Naknek River. In 2003, Big Creek had a Chinook salmon return of 10,063 fish. It originates in the mountains south of Brooks Lake in Katmai National Park and flows northwest for approximately 60 km before joining the Naknek River. Big Creek is a clear water stream with a maximum width of approximately 15 m and riffle-run-pool format (Anderson et al. 2004). Methods As part of a larger mark-recapture/radio telemetry study taking place during the summer of 2003, Chinook salmon were sampled from the main stem of the Naknek River and Big Creek for age, sex, and length data. For this analysis, fish from the main stem of the Naknek River and Big Creek will be considered as different stocks, which was defined by Cushing (1981) as a group of fish spawning in a particular lake or stream at a particular season, which to a substantial degree do not interbreed with any other such group. Because of large river size (prevents the use of a weir) the main stem fish were sampled by drifting 14 and 19.1 cm (stretched mesh size) multifilament gillnets through known pre-spawn (sexually mature) staging concentrations in the upper river between Rainbow Bend and Shawbeck’s Cabin (Figure 1). Fish were sampled for mid-eye to fork length (mm) and sex and returned to the river. Big Creek fish were sampled at a nontraditional resistance board weir approximately 35 km upstream from its confluence with the Naknek River (Anderson et al. 2004).5 Data analysis Healy and Heard (1984) used previously collected data (from 1968, provided by Alaska Dept. of Fish and Game) to develop a regression line to estimate fecundity based on length for the Nushagak River Chinook salmon stock. The Nushagak River is approximately 75 km northwest from the Naknek River. I will use the parameter estimates from Healy and Heard (1984) to estimate fecundity (F) of the Naknek River and Big Creek stocks of Chinook salmon. The regression calls for the length of the fish to be measured from the posterior margin of the orbit to the end of the hypural plate (POH). Fecundity can be calculated with the following equation: F = 1.63 * L1.3108 (1) where F = fecundity and L = POH in mm. However, the length data was collected as mid-eye to fork length (MEF). These lengths can be transformed from MEF to POH by using a simple equation as described by Healy and Heard (1984). All lengths are in mm. The transformation equation is: POH = 0.859 * MEF + 25.7 (2) Fecundity


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