The ecology of salmon and trout in lakes

Transcription

1 The ecology of salmon and trout in lakes 1. Lakes are important habitats for many salmonids 2. Lake users may be: Lacustrine-Use lakes exclusively (e.g. lake trout) Adfluvial-Use stream habitats for spawning but move to lakes for rearing (e.g., cutthroat, rainbow trout, brook trout, Dolly Varden) including summer feeding and overwintering. Lakes are also the primary (but not exclusive) rearing habitat for sockeye salmon prior to seaward migration.

2 Lake Terminology Limnetic (photic zone) Littoral (near-shore area with aquatic plants) Benthic (aphotic lake bottom) The distribution of light is a very important feature of lakes.

3 Water above 4 C becomes lighter as it warms, so lakes are stratified in summer, and mix in spring and fall. Depending on temperature, they may stratify in the winter (freeze) or not Epilimnion (surface layer) Thermocline Hypolimnion (below thermocline)

4 Ecology of sockeye in lakes Foraging and growth Prey choice Growth: interaction between food and temperature Survival Diel vertical migrations Pursuit of prey Thermoregulation and digestion efficiency Predator avoidance

5 Foraging by sockeye in lakes In lakes, sockeye initially prey on insects in the littoral zone but primarily feed on crustacean zooplankton in the limnetic (open water) zone. Zooplankton are not filtered but rather are encountered, detected, pursued, captured, and consumed one at a time.

6 Sockeye shift habitats as they grow and the season progresses Beach seine sampling (littoral zone) Frequency Fish caught at the lake shore (in the littoral zone) Lake Aleknagik, AK Tow net sampling (limnetic zone) Frequency Fish caught at the open lake (in the limnetic zone) Sockeye length(mm) R. Hovel, unpublished

7 Typical sockeye salmon lake ecosystem Trout, charr, pikeminnow Loons, terns, mergansers Sockeye salmon 3-spine sticklebacks Herbivorous zooplankton Invertebrate predator on zooplankton Phytoplankton Nutrients

8 Juvenile sockeye salmon, about 28 mm at lake entry

9 Threespine stickleback, Gasterosteus aculeatus

10 Daphnia, a genus of Cladoceran zooplankton Cyclops, a genus of Cyclopoid copepod

11 Neomysis mercedis, a freshwater shrimp, preys on zooplankton such as Daphnia

12 Plankton are preyed upon selectively: Large, visible, slow prey are eaten more often than would occur by chance. Why? Visibility? Preference? Catchability? Zooplankton species and size composition in lakes are heavily influenced by planktivorous fishes Most lakes in which sockeye salmon rear are nutrient-poor and growth is slow

13 Sockeye typically enter the lake as temperatures and zooplankton are increasing, not at the peak of prey availability (e.g., Lake Washington) 7 Zooplankton Temperature 25 Zooplankton/liter Sockeye fry Temperature 0 J F M A M J J A S O N D Month 0

14 Smolt size is affected by date of entry by fry and summer growth; little growth occurs in winter 120 Mean length (mm) Washington Babine 0 1-Mar 1-Jul 31-Oct 2-Mar 2-Jul Sampling data

15 Lakes vary greatly in potential for growth (e.g., the average size of age-1 sockeye salmon smolts) 25 Frequency Temperature and prey base (resulting from production and competition) largely determine growth in lakes Mean Length (mm)

16 Variation in growth among years in a given lake is often density-dependent, though temperature can also play a role. 70 Fry length (mm) Lake Aleknagik, Alaska Thousands of adult sockeye salmon

17 Density-dependent growth and life history of sockeye salmon Fry stocked into Leisure Lake, AK (Koenings and Burkett 1987) fry stocked (millions) % age 1 smolts smolt age 1 (g) weight age 2 (g) smolt biomass

18 Sockeye eat zooplankton Zooplankton eat phytoplankton Nutrients often limit phytoplankton So, do nutrient concentrations affect the entire food web? Does commercial fishing rob lakes of critical nutrients from salmon carcasses?

19 Effects of fertilizing British Columbia coastal lakes with N and P Variable Primary production (mg C/m 2 /d) Zooplankton biomass (mg dry/m 3 ) Sockeye smolt weight (g) Before treatment After treatment % increase

20 Effects of Fertilization on Zooplankton and Sockeye Salmon Growth and Development Pre-Fertilization Post-Fertilization Herbivorous zooplankton 38,000/ m 2 63,000/ m 2 Daphnia Size 0.75 mm 0.90 mm % age 1 smolts 5% 35% Age 1 smolt weight 5 g 11 g Age 2 smolt weight 7 g 20 g Packers Lake, AK: Koenings and Burkett 1987

21 Thus, growth is affected by the quantity of prey, itself a function of the levels of nutrients in the lake and the grazing pressure of salmon and their competitors. But, what is the interaction between food and temperature in controlling growth?

22 Specific growth rate (% weight/day) Starved 1.5% 3% 4.5% 6% Excess Lethal temperature Temperature o C J. R. Brett

23 Water temperature can affect fry size through date of emergence and growth (e.g., Iliamna Lake) Fry length (mm) on 1 Sept r 2 = June 1-15 water temperature

24 Summer growth of fry can determine whether or not they migrate to sea the next year or not (Iliamna Lake) % age 1 smolts r 2 = mean fry length on 1 September

25 Nutrients (N, P) Local geology and hydrology Predation risk + - Food - - Number of spawning adults + Intraspecific competition Interspecific competition Latitude - + Early date of ice breakup Elevation - Temperature + Air temperature, solar radiation Variation among lakes Variation among years

26 Variation among years in the estimated fry-to-smolt survival in Chilko Lake, B.C. 6 Number of years Mean Chilko among Lake, years: B.C. 29% survival sockeye % fry to smolt survival

27 Diel (24-h period) vertical migration Juvenile sockeye salmon typically school in deep water during the day, rise to nearsurface waters at dusk to feed, and either Descend back to deep water until the next night Descend to intermediate depths for the night, ascend again at dawn, then go deep for the day Stay at intermediate depths all night, then descend at dawn

28 Babine Lake: peaks at dusk and dawn, intermediate depth at night Depth Depth Lake Washington: peak at dusk, return to deep water Day Dusk Night Dawn Day

29 Diel vertical migration: Hypotheses Fish move to the surface and back down to follow movements of prey (i.e., diel vertical migration of zooplankton). Fish feed at the surface but digest their prey in deeper water for energetic efficiency. Fish spend the day in deep water to reduce predation risk and forage near the surface (where zooplankton are found) at dusk and dawn because the fish need light to feed.

30 Diel vertical migration: How do we explain the general phenomenon, and the variation among lakes? Hypothesis: Fish move to the surface and back down to follow movements of prey (i.e., diel vertical migration of zooplankton). This is plausible but not consistently true. Sockeye salmon vertically migrate in many lakes where their prey do not.

31 Diel vertical migration: How do we explain the general phenomenon, and the variation among lakes? Hypothesis: Fish feed at the surface but digest their prey in deeper water for energetic efficiency. This may explain why fish do not spend the night at the surface but it does not explain why they go so deep in the day, and why they also migrate when the lake is mixed.

32 Diel vertical migration: How do we explain the general phenomenon, and the variation among lakes? Hypothesis: Fish spend the day in deep water to reduce predation risk and forage near the surface (where zooplankton are found) at dusk and dawn because the fish need light to feed. This is strongly supported. Fish go deeper in clearer lakes than in less clear ones, and in very turbid lakes they may not migrate or even move up during the day. In clear lakes, the timing of migration is closely linked to light levels.

33 Diel migration of sockeye salmon in Little Togiak Lake, Alaska predators sockeye Depth (m) Incident light Light level Time of day (h) Scheuerell and Schindler 2003

34 Diel migration: Unified theory Daytime depth is determined by predation risk, related to water clarity Fish forage near the surface at dusk because light is needed to see the zooplankton, which are near the surface Night-time depth is determined by water temperature, for efficient digestion of prey Dawn feeding is common but if prey are very abundant then the daily ration can be obtained by merely feeding at dusk Clark and Levy

35 Photo: Greg Ruggerone Use of lakes by adfluvial trout

36 In many systems, trout (rainbow, cutthroat, brown, charr) spawn in streams and juveniles rear there for several years before moving to the lake. In the lake they commonly feed in the littoral zone, mostly on invertebrates, until they grow large, at which time they feed mostly on fish in the limnetic zone. % of fish in the habitat Lake Washington cutthroat trout Fork length (mm) limnetic littoral creek

37 Length of fish prey as a function of predator length: Lake Washington cutthroat trout. Trout can eat fish about 40% of their length. 160 Prey fish length (mm) Sockeye Unid. salmonids Smelt Sculpin Stickleback 20 Nowak et al Predator length (mm)

38 Ecological and evolutionary divergence of trout in lakes Intraspecific diversity occurs within lakes Lakes are often isolated and geologically new Interspecific competition is low (empty niches) Intraspecific competition high

39 Þingvallavatn, Iceland

40 Arctic charr diversity in Þingvallavatn 5 cm Large benthic form Dwarf benthic form Large piscivore Pelagic planktivore (Snorrason & Skúlason, 2004)

41 Environment induces divergence through frequency-dependent selection High food availability, low intraspecific competition Resource consumption Low food availability, high intraspecific competition Trait value

42 Ferox Sympatric forms of brown trout, e.g., in Lough Melvin, Ireland Gilaroo Sonaghen Artwork by Rod Sutterby

43 Summary: The ecology of salmonids in lakes is as varied as the species and lakes themselves. The lakes supporting sockeye salmon are often unproductive, and the juveniles growth is controlled by temperature and food availability. They are risk-averse, reducing foraging opportunities to minimize predation risk with diel vertical migrations. Trout occupy lakes opportunistically and can grow rapidly and to large size, depending on food, temperature and competition. In recently de-glaciated lakes with few fish species, trout often diverge into genetically and ecologically distinct forms, also called eco-phenotypes.