Cumulative effects of piscivorous colonial waterbirds on juvenile salmonids: A multi predator-prey species evaluation

Abstract

We investigated the cumulative effects of predation by piscivorous colonial waterbirds on the survival of multiple salmonid (Oncorhynchus spp.) populations listed under the U.S. Endangered Species Act (ESA) and determined what proportion of all sources of fish mortality (1 –survival) were due to birds in the Columbia River basin, USA. Anadromous juvenile salmonids (smolts) were exposed to predation by Caspian terns (Hydroprogne caspia), double-crested cormorants (Nannopterum auritum), California gulls (Larus californicus), and ring-billed gulls (L. delawarensis), birds known to consume both live and dead fish. Avian consumption and survival probabilities (proportion of available fish consumed or alive) were estimated for steelhead trout (O. mykiss), yearling Chinook salmon (O. tshawytscha), sub-yearling Chinook salmon, and sockeye salmon (O. nerka) during out-migration from the lower Snake River to the Pacific Ocean during an 11-year study period (2008–2018). Results indicated that probabilities of avian consumption varied greatly across salmonid populations, bird species, colony location, river reach, and year. Cumulative consumption probabilities (consumption by birds from all colonies combined) were consistently the highest for steelhead, with annual estimates ranging from 0.22 (95% credible interval = 0.20–0.26) to 0.51 (0.43–0.60) of available smolts. The cumulative effects of avian consumption were significantly lower for yearling and sub-yearling Chinook salmon, with consumption probabilities ranging annually from 0.04 (0.02–0.07) to 0.10 (0.07–0.15) and from 0.06 (0.3–0.09) to 0.15 (0.10–0.23), respectively. Avian consumption probabilities for sockeye salmon smolts was generally higher than for Chinook salmon smolts, but lower than for steelhead smolts, ranging annually from 0.08 (0.03–0.22) to 0.25 (0.14–0.44). Although annual consumption probabilities for birds from certain colonies were more than 0.20 of available smolts, probabilities from other colonies were less than 0.01 of available smolts, indicating that not all colonies of birds posed a substantial risk to smolt mortality. Consumption probabilities were lowest for small colonies and for colonies located a considerable distance from the Snake and Columbia rivers. Total mortality attributed to avian consumption was relatively small for Chinook salmon (less than 10%) but was the single greatest source of mortality for steelhead (greater than 50%) in all years evaluated. Results suggest that the potential benefits to salmonid populations of managing birds to reduce smolt mortality would vary widely depending on the salmonid population, the species of bird, and the size and location of the breeding colony.

Estimating Cause‐Specific Mortality and Survival of Juvenile Fall Chinook Salmon: An Investigation of Avian Predation across Large Spatial Scales

“…It should be noted, however, that fall Chinook Salmon, which are smaller in size than most other salmonid smolts (Quinn 2005), are within the size range documented to occur in the diets of common mergansers and grebes, and studies regarding the predation impacts of these pursuit-diving species on smolts are currently lacking. As a result, the impact of other piscivorous waterbirds on URB Chinook Salmon smolt mortality is unknown but is likely less than those of CATE, DCCO, AWPE, and LAXX, the principal avian predators of actively migrating salmonid smolts documented by this and other studies (Collis et al 2002;Roby et al 2003;Wiese et al 2008;Evans et al 2016Evans et al , 2022bPayton et al 2019). It should also be noted that wild juvenile URB Chinook Salmon reside in and otherwise rear in the HR for several months before being PIT-tagged and prior to out-migrating in June.…”

“…For instance, Evans et al. (2012, 2016, 2022b) documented significantly higher rates of predation on steelhead compared to Chinook Salmon in the Columbia River basin. Evans et al.…”

“…(2015) and Evans et al. (2019, 2022b) were used to estimate colony‐specific detection probabilities and predator‐specific (CATE, DCCO, LAXX, and AWPE) deposition probabilities.…”

“…Informative prior distributions that were used to inform the values of were taken from Evans et al. (2022b).…”

“…To estimate detection probabilities, as part of independent studies, PIT tags were intentionally sown in nesting areas by researchers prior to, during (when possible), and after the breeding season each year at each of the colonies included in this study (Evans et al. 2012, 2019, 2022a, 2022b). Recoveries of sown tags during scanning efforts after the nesting season were then used to model the probability of detecting a tag that was deposited on the colony during the breeding season (see also Predation and Survival Estimation below).…”