Salish Sea Marine Survival Project

25 WHAT WERE OTOLITHS USED FOR DURING SSMSP? Otolith Growth Rings: Annual Growth and Age Otoliths are a very reliable tool for determining the age of a fish. In Pacific Salmon annual growth shows up as two distinct bands: a wide ring of dark material (spring and summer growth) and a narrow ring of relatively clear material (fall and winter growth). Biologists can estimate the age of the salmon by counting annuli similar to the way one can count rings on a tree to determine its age. During the Salish Sea Marine Survival Project, many studies used juvenile otoliths to assess size, growth rates and size at estuarine entry of juvenile Chinook and Coho. Other studies looked at returning adult otoliths and used back calculations to assess the influence of size–selective mortality on previous life history stages (e.g., the rela- tionship between size at estuary entry or growth rates of juveniles and subsequent survival). Otolith Microchemistry: Where Did the Fish Live? The chemical composition of each layer of an otolith depends on what was in the water around the fish at the time the calcium carbonate was laid down. Quantities of elements such as strontium and calcium can be measured from core to the edge of the otolith to reveal the migration history of the fish, since different environments have different amounts of these rare elements which get incorporated into the otolith. This means that the different parts of the otolith have a unique chemical signature that researchers can use to determine where salmon have been over their lifetime. Also, because different river systems many contain different compositions of these chemical elements, microchemistry techniques can be used to discriminate among stocks or even determine lake or river of origin. Otolith Shape and Size: Fish Identification and Predation Studies Otoliths in different species of fish have a distinct shape which can often be diagnostic for that species. Therefore, biologists can look at the shape and size of undigested otoliths in stomach contents or droppings from other predators (avian, fish or mammalian) to reconstruct the species of fish eaten by a predator. Additionally, the size of the otolith can be used to determine the size of the fish eaten. SSMSP Studies Using Otoliths During SSMSP, large numbers of otoliths were collected from juvenile and adult salmon to allow us to examine estuarine habitat use, life-history patterns and the relationship between early marine growth and survival of Chinook Salmon. Otoliths from adult Chinook returning to systems have been compared with the otoliths of the juveniles from those same systems to determine which juvenile life history contributed to adult returns. This included determination of whether smaller fry or larger parr were contributing more or less to adult returns, and whether growth rates, estuary residence or other factors were also important to survival. Studies done by Lance Campbell and colleagues (Washington State, Department of Fish and Wildlife) have shown that small Chinook fry (35-60 mm) survive to contribute to several populations in Puget Sound and to the Cowichan River in Canada. As adults, these small fry appear to make up as much as 35% of the returning adults to many systems, with the remainder of the returns derived from larger juveniles that leave the systems as larger parr (>60 mm). They also noted that survival of the small fry appears to be greater from systems with intact and less compromised estuaries, which are important rearing habitat for these small fish. This possible relation- ship between survival and estuary habitat condition could also be related to pathogen and contaminant exposure in the more degraded estuaries. Other studies by Lance have also shown that Coho Salmon that utilize estuarine rearing strategies are important to Coho returns in Puget Sound, an important finding given that the traditional estimates using stream habitat capacity will often underestimate the importance of the estuary-rearing components. As part of SSMSP and in some of our ongoing studies, such as the Adult Chinook and Coho diet studies spear- headed by Will Duguid at the University of Victoria, otoliths have been collected from stomach contents of salmon to provide information on the species consumed by the various Pacific Salmon. NEXT STEPS SSMSP results have focused on the importance of connected and healthy estuaries to both Chinook and Coho life history diversity and survival. We suggest that more work needs to be done to assess the variations in estuary habitat usage, residence and growth of Coho and Chinook in additional Canadian systems as in the Cowichan River. Understanding the length of residence and growth of juvenile salmon in different BC estuaries, and most importantly, the contribution of these fish to the adult populations returning, will help guide and focus future habitat restoration efforts.

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