Fish Conservation and Management

Transcription

1 Fish Conservation and Management CONS 486 Applied life history: individual growth, population growth, competition, predation Ross Chapter 3

2 Applied life history Age and size at maturity Growth Competition individual growth population growth Predation Natural Humans

3 Major theme: Linking science to conservation & management Physiology Behaviour Population ecology Ecosystem ecology Habitat data (limnology, oceanography) Life history Basic science Applied science Fisheries exploitation data Applied life history data Human dimensions: socioeconomic data Protecting populations & habitats Restoring populations & habitats Conservation Management Harvest regulations Managing fisheries & habitats

4 Energy and fish growth

5 Age and size at maturity Bcsalmon.ca Vast differences among species! But within a species, age at maturity is correlated with growth rate Fast growth means lower age at maturity E.g., Pink salmon mature in half the time of other salmon and they have the fastest ocean growth rates Age of Maturity 5-7 yrs 4-6 yrs 3-5 yrs 3-5 yrs 2 yrs

6 Salmon jack and jills are precocial maturers Mature 1 year earlier These fish have highest ocean growth rates Typical sockeye adults (mature at 4 yrs) Sockeye jacks (mature at 3 yrs)

7 Mortality and size at maturity If mortality of young fish is excessively high Then selection favours early maturation (i.e., r-selection; opportunistic!) If mortality caused by the act of spawning is high Then selection favours maturation as late as possible to enable more time for growth (i.e., K-selection; equilibrium!) Bigger means more eggs!

8 Growth: Energy and fish growth Fish growth is indeterminate Think swimming trees but far more interesting! Figure 3.2 Ross birth death

9 Growth: Energy and fish growth Growth increases rapidly in early development immature birth death

10 Growth: Energy and fish growth Growth decreases as fish begin to mature and gonads develop mature immature birth death

11 Growth: Energy and fish growth Senescence: energy required to sustain routine body function, less required for reproduction mature senescent immature birth death

12 Growth: Life stage growth Individual growth variation exists within each age class Becomes very evident as fish get older E.g., fast growing 3 yr can have same lengths as slow growing 5 yr Young of year 1 yr olds 2 yr olds 3 yr olds 4 yr olds 5 yr olds 5+ yr olds

13 Competition at two levels 1. Individual: competition can influence growth 2. Population: competition can control population size too! Bluebison.net

14 Individual growth and competition Competition: When population abundance is high And competition for food is high The available ration will be reduced (reduced per capita food intake) Thus individual body sizes decreases as ration is reduced Referred to as Density Dependent Growth (DDG) which occurs at environmental carrying capacity I.e., when biomass at maximum, such that no further biomass could be supported by the available food If ration is high because of low abundance and high food per capita (below carrying capacity) individual growth can be Density Independent (DIG) i.e., body size more affected by abiotic factors like ocean temperature, ph, etc. Can fish in one population be affected by DDG in one year, then DIG in another year??

15 Example with 8 Fraser River sockeye populations Length of females declines with high abundance (above M salmon) Below 80 M no relationship between length and abundance Carrying capacity ~ M?? S. Cox MSc data Sockeye Abundance in Gulf of Alaska (millions)

16 Caveats: DDG DDG may only be evident in years when abundance is relatively high DDG is often hard to detect if fisheries exploitation is high (WHY?) Because their numbers are kept low due to high fishing mortality In those cases, DIG factors may be most important Note density independent factors are always present, but that the relative importance can vary

17 Population size and competition Goliath grouper and cigar minnows; Smithsonian

18 Intra-specific competition Intra-specific competition can reduce an individual s ability to access food or physical habitat - this can have affects on population abundance. When food is limited: an individual s size can be reduced and this reduces fecundity fish may have difficulty acquiring enough reserve energy to survive periods of starvation (e.g. juveniles in winter, anadromous spawning migrations) When physical habitat is limited: some individuals may not access spawning areas refuge areas may not be available to all that need them (e.g. areas where juveniles can escape predators)

19 Predation can influence population size too!

20 Most predatory fishes attack the side or head of their prey like above. A few, like bluefish below, strike from behind and bite pieces off. Note the razor like teeth on the bluefish

21 Predation It s a fish eat fish world But generally a fine balance between predators and prey Predators can keep their prey at low abundances Cannibalism can occur Rarely does predation result in extirpation Can occur with new predator introduced into a predator-less system

22 Example predatory Bull Trout eating sockeye smolt prey Age-1 V4 tags

23 Bull trout stomach 5 cm

24 What types of smolts do bull trout eat?

25 Bull Trout eat smaller fish = = Population Predated

26 IHN virus relative load Bull Trout selecting smolts with higher loads of IHN virus 2014 Minimum positive threshold 93% 28%

27 Natural predators Natural predators tend to remove the smallest, weakest individuals of prey Therefore, selection for fast growth rates in prey Unexploited (i.e., unfished ) pops usually have: A high proportion of old fish A few very large individuals Slow individual growth rates among predatory fish Low rates of annual mortality among predatory fish Large individuals can control population size by increasing competition (and even cannibalism) Results in high competition among small fish

28 Human predators Human predators tend to remove the largest, strongest individuals E.g., sport fishers keep big ones and release the small ones (Donaldson et al Fisheries) E.g., commercial fisheries gear target largest fish (Olden et al Glob Ecol Biogeo) Tigerfish

29 Fisheries exploitation With the start of exploitation: The few real big fish disappear Nature of density dependence changes Cannibalism no longer main issue Competition still occurs but less intense Length:age frequency distributions often shift toward smaller and younger fish Individual growth rates of surviving fish could increase due to reduced competition This could be sustainable but only at moderate levels of exploitation

30 Linking science and management Interesting example of human predation: fisheriesinduced selection! Handford et al (1977) one of the first to examine effects of commercial fishery on life history traits Examined whitefish growth from Lesser Slave Lake, Alberta in relation to heavy gillnet exploitation over 30 years By 1965 the fishery had to be closed or severely reduced

31 Lake whitefish (Coregonus clupeaformis)

32 Lesser Slave Lake, Alberta

33 Lesser Slave Lake: serious about fishing

34 VERY serious about fishing! $250,000 annual tournament!

35 Effects of human predators on prey: Artificial selection opposite natural selection Handford et al J Fish Res Bd Canada

36 During early years of the fishery, age classes were easily observed and discrete, reduced competition leads to increasing sizes within year classes But towards the end of the intensive fishery: Average body size was decreasing Differences in age class sizes was diminishing Fish were becoming similar sized Young fish were absent Possibly recruitment failure or the fact that young fish were too small to be captured in sampling gear

37 Gillnetting removed the highest conditioned (fattest) fish Young age classes were disappearing (see last figure) partly because of reduced juvenile production from low fecundity and harvesting juveniles Handford et al J Fish Res Bd Canada

38 Whitefish study: conclusions As fisheries exploitation increases Competition for food can decrease Results in increased individual growth rates Fisheries remove older (& larger) individuals Decrease in mean age; higher % of younger fish Results in exploitation affecting mean age and growth rate Can influence the ability of a population to cope with further exploitation Over time, heavy gillnetting pressure removes large fish causing strong selection for small body sizes Smaller fish have low fecundity Population would crash if heavy fishing continued on small bodied fish

39 Final words - Abiotic factors can also affect population size E.g., climate events can cause mortality to eggs or juveniles, or displace fish into inhospitable areas Storm over Lake Erie