Changes in Algal Species Composition Affect Juvenile Sockeye Salmon Production at Woss Lake, British Columbia: A Lake Fertilization and Food Web Analysis

Abstract

In recent decades, the abundance of sockeye salmon Oncorhynchus nerka in the Nimpkish River watershed (Vancouver Island) has declined by more than 80%. To improve sockeye salmon abundance, one of three Nimpkish nursery lakes (Woss Lake, 13.6 km 2 ) was fertilized; a second (Vernon Lake, 8.4 km 2 ) was maintained as an unmanipulated reference. For 3 years (2000)(2001)(2002), we assessed changes in phytoplankton, zooplankton, and juvenile sockeye salmon and then used food web analysis to model the fate of fertilizer phosphorus (FP) as it moved up the food web to juvenile sockeye salmon. As the experiment progressed, we found two distinctly different results. During the first 2 years (2000 and 2001), fertilization was associated with higher concentrations of P, chlorophyll, and phytoplankton. However, because most of the algae were inedible diatoms (principally Rhizosolenia eriensis), only 8% of the algal standing stock was available to zooplankton and less than 0.1% of the FP was incorporated into sockeye salmon biomass via limnetic pathways. The result was that juvenile sockeye salmon growth rates in the treated and control lakes were not significantly different. During the third year (2002), a bloom of the diatom Leptocylindrus increased Woss Lake edible algal biomass by a factor of 7; zooplankton production tripled, and juvenile sockeye salmon production increased by 19%. Consequently, Woss Lake juvenile sockeye salmon grew twice as fast as they did in untreated Vernon Lake, and juvenile sockeye salmon fall weights were 50% higher than those recorded before the lake was fertilized. We conclude that for north Pacific coastal lakes, bottom-up effects resulting from changes in phytoplankton species composition can profoundly alter rates of juvenile sockeye salmon production. Regulation of algal species composition through manipulations of fertilizer composition and application timing might be used to reduce blooms of nuisance algae and increase age-0 sockeye salmon yield.

Habitat Manipulations Confound the Interpretation of Sockeye Salmon Recruitment Patterns at Chilko Lake, British Columbia

“…The reasons for this are not clear although McQueen et al (2007) have previously noted the potential for variable smolt production outcomes given diff erential responses of algal taxa to consecutive years of similar inorganic fertilizer additions at Woss Lake, BC.…”

Density-Dependent Growth in Juvenile Sockeye Salmon (OncorhynchusNerka)

“…Woss Lake (50 O (surface area of 837 ha, mean depth 62 m, average TP = 2.9 g L -1 ) drains into the Sebalhal River which also flows into the Nimpkish River [31]. The Woss River system produces approximately 25% of total Nimpkish system sockeye salmon.…”

“…We also measured O. nerka densities, lengths, weights, biomasses, stomach contents and estimated their preyspecific rates of consumption. We then calculated prey consumption by nerkids as percent daily species-specific zooplankton production to draw inferences about whether consumption exceeded production [31]. This allowed us to simulate the effects of changing planktivorous fish numbers and to calculate the density threshold at which consumption by fish > production of their zooplankton prey.…”

“…Fish were processed for lengths, weights and stomach contents. Survey protocols including stomach content analysis are described in McQueen et al [31]. Our goal was to calculate June-October rates of consumption by the fish as a percentage of biomass and production by each species of zooplankton prey.…”

“…Also in Lake Washington during April-November, juvenile sockeye plus Neomysis mercedis (Holmes) consumed only 10% per month of Daphnia biomass [29,30]. In British Columbia's Woss and Vernon lakes on Vancouver Island, during 2000-02, juvenile sockeye salmon consumed an average of < 0.5% d -1 of the zooplankton standing stock [31].…”