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2. Fishing effects on populations and communities

2. Fishing effects on populations and communities. Fishing effects on populations and communities. Vulnerability to fishing : Behaviour – Catchability, susceptibility Intraspecific effects : Age and size structure Reproduction Genetic structure Community effects : Diversity

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2. Fishing effects on populations and communities

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  1. 2. Fishing effects on populations and communities

  2. Fishing effects on populations and communities Vulnerability to fishing: Behaviour – Catchability, susceptibility Intraspecific effects: Age and size structure Reproduction Genetic structure Community effects: Diversity Community structure Size structure

  3. Bottom trawling in North Sea • Average annual trawling frequency of the sea bed by the total Dutch beam trawl fleet in the 4-year period between 1993 and 1996 as estimated on a 1x1 nautical mile scale. • 30% of the seabed is trawled 1-2 x per year • 10% of the seabed is trawled more than 5 x per year From Rijnsdorp et al. 1998

  4. Ghost Fishing

  5. Cyanide fishing

  6. Muroami fishing

  7. Dynamite fishing

  8. Vulnerability to fishing:Catchability → F = fishing mortality, C = catches, B = average biomass, f = nominal effort (fishing power), and q = catchability (fishing efficiency) Catchability (q) is defined as the relationship between the catch rate (CpUE) and the true population size (B). So the unit of catchability is fish caught per fish available per effort unit and per time unit.

  9. Vulnerability to fishing:Catchability • The probability of a fish being caught at any time depends on several factors, which broadly can be grouped into biological and technological factors: • The biological factors include: • fish availability on the fishing ground, • fish behaviour (incl. towards the fishing gear), • the size, shape, and external features of the fish, • where some of these factors again are depending on season, age, environment and other species.

  10. Vulnerability to fishing:Catchability Technological factors include: - gear type, design, size, colour, and material, - gear position, duration, and handling, - experience of the fisherman As both the unit and the different notations epitomise, the catchability coefficient (alias efficiency, or fishing power, or probability of a fish being caught), is therefore a composite and very complicated factor. ‘Fish catchability’ normally refer to changes in fish behaviour. ‘Fishing efficiency’ refers to fishing practises or relative fishing power.

  11. catchability (q) Effort (f) Fishing mortality (F) Better methods Increasing these is development Fishing mortality (F) So while we ‘manage’ and ‘develop’ the fishing mortality stays the same. More of the same Decreasing these is management

  12. Behaviour: Many fish shoals

  13. Shoaling protects against predators But not all!

  14. Behaviour:Fish Aggregation Devices (FADs) ‘Home made’ FAD used on the Nippon Maru

  15. Fish Aggregation Devices (FADs) Small pelagic fish Algae fixed to FAD Juvenile fish Large predators

  16. Effects of FADs - catchability Effects of FADs in La Reunion Island http://www.spc.int/coastfish/News/FAD/FAD3.pdf

  17. FAD??

  18. Life history and vulnerability

  19. Predation vs fishing mortality.. .. is almost exactly opposite Predation mortality Fishing mortality Age (years) From ICES (1997).

  20. ..and this is what happens: Median age-at-maturation (sexes combined) of Northeast Arctic cod based on spawning zones in otoliths(from Jørgensen, 1990).

  21. But we know that – we even use it as a sign of fishing Age and size structure changes under selective fishing to younger and smaller individuals. effort

  22. Intraspecific effects: age and size structure • As age and size structure changes • under selective fishing to younger and smaller individuals there will be • a decrease in: • size (age) of maturity • fecundity, • egg quality • egg volume, • larval size at hatch, • larval viability, • food consumption rate, • conversion efficiency, • growth rate.

  23. Intraspecific effects: age and size structure – density dependence Abesamis and Russ 2005 MPA in the Philippines on Apo Island

  24. The r-K selection principle:genetic changes? Increased juvenile mortality = K-selection Increased adult mortality = r-selection Slope = total mortality rate Z = r Abundance (Log N) Age (size) Kolding (1993) K-selection: Stable environment, biotic mortality (predation) – predictive, size selective r-selection: Unstable environment, abiotic mortality – non-predictive, non-selective

  25. Size selection = genetic changes Increased mortality on: Small Random V Large Mean individual weight at age for six harvested populations after 4 generations. Circles, squares, and triangles represent the small-, random-, and large-harvested populations, respectively. After Conover and Munch Science 2002

  26. Effect of size-selective fishing Mortality on: Small Random Large Size selective fishing with large mesh sizes on adults is decreasing mean size and lowering yields We are deliberately inducing r-selection on the stocks. Trends in average total weight harvested (A) and mean weight of harvested individuals (B) across multiple generations of size-selective exploitation. Closed circles represent small harvested lines, open squares are the random-harvested lines, and closed triangles are the large-harvested lines. Conover and Munch 2002

  27. Size selection: genetic changes? • Correlated responses to harvest selection on • (L) Large individuals • (R) Random individuals • (S) Small individuals • egg volume, • size at hatch, • growth efficiency (unlimited food), • growth efficiency (restricted food). After Walsh et al. 2006 R S R S L L

  28. Size selection: genetic changes? • Correlated responses to harvest selection on • (L) Large individuals • (R) Random individuals • (S) Small individuals • % survival (10 days), • consumption rate, • # vertebrae • Forage response time After Walsh et al. 2006 R S R S L L

  29. Community effects: Diversity Extirpation: Local loss of population Sedentary coral reef species Elasmobranchs Relative abundance Baum et al. 2003

  30. Diversity Worms et al. 2006 32 100 320 1000 3200 10000

  31. …and the doomsday predictions based on the extrapolation of regression there will be 100% collapse in the year 2048 Global loss of species from LMEs. Trajectories of collapsed fish and invertebrate taxa over the past 50 years (diamonds, collapses by year; triangles, cumulative collapses). Worm et al. (Science 2006).

  32. Which is most diverse? 3 species of grass ? 2 species of grass + a rabbit ??

  33. Diversity – how measured? Order Family Genus Species

  34. Fishing and diversity Relationship between species richness (± 95%CL) and fishing intensity for groupers on Fijian reefs. After Jennings and Polunin (1997). Except for perhaps first 2 obs no significant difference – same habitats? 0-hypothesis: All reefs have same number of groupers at all times ??

  35. Diversity changes naturally ! • Number of species and relative evenness increase during succession Early succession phase 29 species Filling phase 19 species Late succession phase 35 species

  36. Attributes of ecological succession • Odum (1969) proposed a number of ecological attributes as indicators of the maturing process during succession in an stable ecosystem. In Kariba we could test 8 out these.

  37. The intermediate disturbance hypothesis (Connell 1978) • Here the diversity is a result of a balance between the frequency of disturbances that provide the opportunity for re-colonization, and the rate of competitive exclusion. Disturbances are here seen as catastrophic and density independent events.

  38. What is a disturbance? • "A discrete, punctuated killing, displacement, or damaging of one or more individuals (or colonies) that directly or indirectly creates an opportunity for new individuals (or colonies) to become established". (Sousa 1984). • "Any relatively discrete event in time that removes organisms and opens up space which can be colonized by individuals of the same or different species" (Begon et al. 1990).

  39. What about human induced stress? • Any (anthropogenic) factor that has the effect of increasing the death rate, or decreasing the birth rate. Harvesting, as well as pollution, falls within this definition". (Pimm & Hyman 1987) Is that not the same as a disturbance??

  40. Impacts are not random Fishing has different impact depending on the trophic level. Traits that make species vulnerable co-vary both between and within trophic levels. Body size (size of the circles) of top carnivore species tends to be larger than that of species at lower trophic levels. Range of body sizes and number of species are larger at lower trophic levels. From Raffaelli (2004)

  41. Community structure: Under fishing the processes are reversed Trends in mean growth rate, maximum length, age-at-maturity, and length-at-maturity in the North Sea demersal fish community (Jennings et al. 1999)

  42. Fishing down the food web Pauly et al. (1998) 5 4 Trophic level 3 2

  43. Fishing down the food web Mean trophic level of the landings in North-east Atlantic fishing area

  44. The distribution of biomass by body size follows regular patterns intercept phytoplankton slope zooplankton small fish big fish The biomass size spectre Biomass Size Slope and intercept changes with pressures and drivers Changes in intercept is informative about changes in biomass While changes in slope is informative about mortality pattern Jennings & Blanchard, 2004

  45. Size-spectre in practise Example of size spectre for the North Sea demersal fish community in 1977 and 1993. The loge numbers of fish per loge 10 cm size classes are shown. Note the steeper slope and higher intercept of the fitted relationship in 1993. After Rice and Gislason (1996).

  46. Size-spectre in practise Slopes (a) and intercepts (b) with 95% CL of linear regressions fitted to size spectre for the North Sea demersal fish community from 1977 to 1993. After Rice and Gislason (1996).

  47. Conclusions Fishing has profound effect on populations and communities: Density reduces (not bad to a point) Susceptible species are easy to overfish Diversity and life history traits are altered Man when fishing is a predator: But we are a highly selective predator In general we behave opposite of all other predators This is the main reason for the observed effects! All natural populations are adapted to predation Perhaps we should rethink the way we predate

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