Mark S. Wipfli

Marine Subsidies in Freshwater Ecosystems: Salmon Carcasses Increase the Growth Rates of Stream-Resident Salmonids

Abstract We tested the hypotheses that marine-derived resource subsidies (salmon carcasses) increase the growth rates of stream-resident salmonids in southeastern Alaska and that more carcasses translate into more growth. Five carcass treatments of pink salmon Oncorhynchus gorbuscha (0, 1, 2, 3, and 4 carcasses/m2 or 0, 1.9, 3.7, 5.6, and 7.4 kg wet mass/m2) were replicated six times in once-through artificial channels, then each channel was stocked with three live age-0 coho salmon O. kisutch. The experiment spanned more than 9 weeks: 16 August to 24 October 1998. The body mass and fork length of the young coho salmon significantly increased from carcass additions, but the incremental increases sharply diminished at carcass-loading levels above 1 carcass/m2. Further, in a small stream in which we added salmon carcasses to a cumulative density of 0.54 carcasses/m2, both cutthroat trout O. clarki and Dolly Varden Salvelinus malma grew significantly faster during the 2 months in which carcasses were added (...

Influence of decomposing Pacific salmon carcasses on macroinvertebrate growth and standing stock in southeastern Alaska streams

We compared macroinvertebrate growth rates and standing stock in the absence and presence of meat from Pacific salmon (Oncorhynchus spp.) carcasses in microcosm and natural stream rearing experiments in southeastern Alaska. In microcosm experiments, the presence of salmon meat increased growth rates and standing stock for the shredder Zapada cinctipes and the collector Psychoglypha subborealis, but not the predator Rhyacophila sp., or the scraper Cinygmula sp. In natural stream experiments, the presence of salmon meat increased the growth rate and standing stock of P. subborealis, but increased only the growth rate of Z. cinctipes. Macroinvertebrate responses to inputs of salmon-derived organic material can vary by species, which may reflect their feeding ecology. Macroinvertebrate taxa belonging to the collector functional-feeding group are likely to be important in transferring the effects of spawning salmon to the rest of the food web in southeastern Alaska streams.

Marine Subsidies in Freshwater: Effects of Salmon Carcasses on Lipid Class and Fatty Acid Composition of Juvenile Coho Salmon

Abstract Returning adult salmon represent an important source of energy, nutrients, and biochemicals to their natal streams and may therefore have a quantitative effect on the energy levels of stream-resident salmonids. We tested this hypothesis by constructing simulated streams for coho salmon Oncorhynchus kisutch to which we added 0, 1, and 4 carcasses/m2 (0, 0.71, and 2.85 kg wet mass/m2) of pink salmon O. gorbuscha. After 60 d we evaluated the lipid class and fatty acid composition of rearing coho salmon from the simulated streams; the lipid content and triacylglycerols of the coho salmon increased with increasing carcass density whereas phospholipids decreased. Increased amounts of triacylglycerols accounted for most of the lipid increase. In addition to increasing in concentration, the fatty acid composition of the triacylglycerols also changed with carcass density. Triacylglycerols of juvenile coho salmon from the control streams had significantly higher omega-3 : omega-6 ratios as a result of five...

Restoring Productivity of Salmon-Based Food Webs: Contrasting Effects of Salmon Carcass and Salmon Carcass Analog Additions on Stream-Resident Salmonids

Abstract We tested the hypotheses that salmon carcasses and salmon carcass analogs (dried, processed hatchery salmon) increase the condition factor, production, and whole-body lipid content of stream-resident salmonids and that stream shading affects responses to enrichment. Two enrichment treatments (salmon carcass, salmon analog) and a control, each with and without simulated riparian shading (95% shade), were replicated six times in once-through artificial channels (mesocosms). Each channel was stocked with three live young-of-the-year coho salmon Oncorhynchus kisutch. The experiment spanned 30 July through 18 2002. Production and lipid content significantly increased in both carcass and analog treatments. Condition was generally unchanged. Stream shading significantly reduced the magnitude of production and lipid responses. In addition, in a small stream where salmon carcasses and analogs were added to two separate tributaries, cutthroat trout O. clarki had higher mean condition, production, and lipid...

Effects of Bacillus thuringiensis var. israelensis on Nontarget Benthic Insects through Direct and Indirect Exposure

We investigated direct and indirect (food-chain) effects of Bacillus thuringiensis var. israelensis (B.t.i.) on selected nontarget benthic insects. Sixteen taxa of various Ephemeroptera, Plecoptera, Trichoptera, and Diptera were exposed to B.t.i. directly, and indirectly through ingesting B.t.i.-contaminated food (black flies and conditioned leaves) in experiments that were conducted in artificial and natural streams. Lethal effects and sublethal effects (drift, feeding, growth, emergence success, and time to emergence) were measured up to 35 d following B.t.i. exposure. In addition, we measured larval detachment and decomposition rates, following B.t.i. application, to determine how long B.t.i.-killed larvae remain available to potential consumers. We observed no lethal or sublethal effects on most taxa following direct and indirect B.t.i. exposure. Mortality was recorded with two species, Tipula abdominalis (Diptera:Tipulidae) and Arthroplea bipunctata (Ephemeroptera: Heptageniidae), when immatures were exposed to B.t.i. at dosages considerably higher (>50× and 500×, respectively) than recommended field rates. Acroneuria lycorias (Plecoptera:Perlidae) nymphs exposed to a high B.t.i. dose (100 ppm for 120 min) drifted at higher frequencies than nymphs not exposed. Predators consumed equal numbers of B.t.i.-killed and live black flies, except for one trial where Isoperla signata (Plecoptera:Perlodidae) ingested significantly fewer B.t.i.-killed black flies than live ones, and another trial where Ceratopsyche sparna (Trichoptera:Hydropsychidae) ingested significantly more B.t.i.-killed than live larvae. Shredders switched to consuming black fly larvae once the black flies were killed with B.t.i. Siphlonurus rapidus (Ephemeroptera:Siphlonuridae) nymphs provisioned with B.t.i.-killed larval black flies attained a greater body mass over 7 d than those not so provisioned. No change in adult emergence success or time to emergence was recorded after respective taxa were exposed to B.t.i. B.t.i.-killed black fly larvae remained attached to substrates in artificial streams up to 16 d, until decomposing beyond recognition. In summary, B.t.i. appears harmless to nontarget benthic insects, even when these organisms are exposed to higher than recommended dosages, and when they consume B.t.i.-killed black fly larvae.

Top-down control of invertebrates by Ninespine Stickleback in Arctic ponds

Despite their widespread presence in northern-latitude ecosystems, the ecological role of Ninespine Stickleback Pungitius pungitius is not well understood. Ninespine Stickleback can occupy both top and intermediate trophic levels in freshwater ecosystems, so their role in food webs as a predator on invertebrates and as a forage fish for upper level consumers probably is substantial. We introduced Ninespine Sticklebacks to fishless ponds to elucidate their potential effects as a predator on invertebrate communities in Arctic lentic freshwaters. We hypothesized that Ninespine Stickleback would affect freshwater invertebrate communities in a top-down manner. We predicted that the addition of Ninespine Sticklebacks to fishless ponds would: 1) reduce invertebrate taxonomic richness, 2) decrease overall invertebrate abundance, 3) reduce invertebrate biomass, and 4) decrease average invertebrate body size. We tested our hypothesis at 2 locations by adding Ninespine Stickleback to isolated ponds and compared invertebrate communities over time between fish-addition and fishless control ponds. Ninespine Sticklebacks exerted strong top-down pressure on invertebrate communities mainly by changing invertebrate taxonomic richness and biomass and, to a lesser extent, abundance and average invertebrate size. Our results supported the hypothesis that Ninespine Stickleback may help shape lentic food webs in the Arctic.