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.

Fig 1. Map of study area.

Capture-recapture-recovery locations of tagged juvenile salmonids initially detected passing Lower Granite Dam on the Snake River. Downstream recapture locations include Little Goose Dam, Lower Monumental Dam, McNary Dam, John Day Dam, Bonneville Dam, and a towed net detection system in the upper Columbia River estuary (Net Detector). Recovery locations include bird colonies on Banks Lake Island (BLI), Potholes Reservoir (PTI), Lenore Lake Island (LLI), Island 20 (I20), Foundation Island (FDI), Badger Island (BGI), Crescent Island (CSI), central Blalock Islands (CBI), Miller Rocks Island (MRI), and East Sand Island (ESI). Avian species include Caspian terns, double-crested cormorants, and California/ring-billed gulls. Numbers represent the distance in river km (Rkm) from the Pacific Ocean.

Fig 2. Avian consumption and total mortality of snake river steelhead.

Estimated annual reach-specific and cumulative avian consumption probabilities (colored bars) and total mortality (grey bars) of Snake River steelhead trout smolts during passage from Lower Monumental Dam on the Snake River to the Pacific Ocean. Piscivorous waterbird colony locations include Banks Lake Island (BLI), Potholes Reservoir (PTI), Lenore Lake Island (LLI), Island 20 (I20), Foundation Island (FDI), Badger Island (BGI), Crescent Island (CSI), central Blalock Islands (CBI), Miller Rocks Island (MRI), and East Sand Island (ESI). Avian species include Caspian terns (CATE), double-crested cormorants (DCCO), and California and ring-billed gulls (LAXX). Error bars represent 95% credible intervals for total mortality and avian consumption.

Fig 3. Avian consumption and total mortality of snake river yearling Chinook salmon.

Estimated annual colony-specific, reach-specific, and cumulative avian consumption probabilities (colored bars) compared to total mortality (grey bars) of Snake River yearling Chinook salmon smolts during passage from Lower Monumental Dam on the Snake River to the Pacific Ocean. Piscivorous waterbird colony locations include Banks Lake Island (BLI), Potholes Reservoir (PTI), Lenore Lake Island (LLI), Island 20 (I20), Foundation Island (FDI), Badger Island (BGI), Crescent Island (CSI), central Blalock Islands (CBI), Miller Rocks Island (MRI), and East Sand Island (ESI). Avian species include Caspian terns (CATE), double-crested cormorants (DCCO), and California and ring-billed gulls (LAXX). Error bars represent 95% credible intervals for total mortality and avian consumption.

Fig 4. Avian consumption and total mortality of snake river sub-yearling Chinook salmon.

Estimated annual colony-specific, reach-specific, and cumulative avian consumption probabilities (colored bars) and total mortality (grey bars) of Snake River sub-yearling Chinook salmon smolts during passage from Lower Monumental Dam on the Snake River to the Pacific Ocean. Piscivorous waterbird colony locations include Banks Lake Island (BLI), Potholes Reservoir (PTI), Lenore Lake Island (LLI), Island 20 (I20), Foundation Island (FDI), Badger Island (BGI), Crescent Island (CSI), central Blalock Islands (CBI), Miller Rocks Island (MRI), and East Sand Island (ESI). Bird species include Caspian terns (CATE), double-crested cormorants (DCCO), and California and ring-billed gulls (LAXX). Error bars represent 95% credible intervals for total mortality and avian consumption.

Fig 5. Avian consumption and total mortality of snake river sockeye salmon.

Estimated annual colony-specific, reach-specific, and cumulative avian consumption probabilities (colored bars) and total mortality (grey bars) of Snake River sockeye salmon during passage from Lower Monumental Dam on the Snake River to the Pacific Ocean. Colony locations include Banks Lake Island (BLI), Potholes Reservoir (PTI), Lenore Lake Island (LLI), Island 20 (I20), Foundation Island (FDI), Badger Island (BGI), Crescent Island (CSI), Blalock Islands (BKI), Miller Rocks (MIR), and East Sand Island (ESI). Avian species include Caspian terns (CATE), double-crested cormorants (DCCO), and California and ring-billed gulls (LAXX). Error bars represent 95% credible intervals for total mortality and avian consumption.