Transcript Fish Ecology

Fish Ecology
Species Diversity and Distribution
Growth and Spawning
Population Dynamics
Stocking
Species Diversity & Global Distribution
• Marine biodiversity generally higher than freshwater.
• ~70% of the earth covered by salt water, 1% freshwater
• 97% of all water in the world is ocean (0.009%
freshwater)
• Are there more marine or freshwater species of fish in
the world?
58% of all species marine
41% freshwater
1% both (diadromous, euryhaline)
• More species on large than small continents.
• Higher species richness in tropics. Why?
Lake Species Diversity
• Seasonal stability / variability of lake
habitats influences species presence.
• Abiotic factors that most influence fish
species distribution in lakes:
– Temperature
– Oxygen availability
• Main biotic factor: primary productivity
(i.e., food availability)
Four groupings of fish communities based on lake
temperature regime and trophic status.
Stream Species Diversity
• Number of species increases as stream order increases.
– Different species / predator types become more predominant
due to changes in abiotic/biotic features of river habitat.
– Relates to changes in flood frequency, temperature regime,
substrate, riparian canopy, etc.
• Functional groups (feeding strategies) change with increased stream
order. What are these strategies?
– Carnivores:
• Piscivores
• Benthophages
• Zooplanktivores
• Epifauna eaters
• Parasites
– Omnivores
– Herbivores
– Detritivores
Spawning, Hatching & Dispersal
•
Broadcast spawning
–
•
Meroplanktonic (not seen in exclusively lotic species)
Benthic spawning
–
–
–
•
Brood hiding (trout)
Brood guarding (cichlids)
Nest spawning (bass, sticklebacks)
Internal bearers: internal fertilization.
–
Oviparity: egg-laying fishes. Very little embryonic development
before eggs are laid.
–
Ovoviviparity: embryos develop internally, but only receive
nutrition from yolk-sac.
–
Viviparity: embryos develop internally, mother provides
additional nutrition after yolk sac used up. Also some instances
of in-utero cannibalism.
Juvenile Development and
Growth
• Transition from larvae to juvenile stage can involve a
dramatic metamorphosis, but changes generally more
subtle:
– Appearance of fully formed fins
– Organ systems fully formed (or nearly so)
– Juvenile period lasts until first onset of sexual maturity (initial
development of gonads)
• Growth: G = C – (R + E)
– Somatic (body) growth is a function of the balance between
consumption/assimilation, respiration rate, and waste excretion
Factors Affecting Growth
• Temperature: dependent on tolerance range of species.
Growth increases with temperature to a point, then falls
off as temperature increases further
• DO: decrease in DO associated with decrease in growth
rate
• Salinity: Important in euryhaline / diadromous fishes
• Food abundance, competition (inter- and intraspecific),
Growth for Reproduction
• Gonadal development occurs prior to spawning, claims a
significant percentage of ingested nutrition
– Testes: up to ~ 12% of male body weight
– Ovaries: up to ~70% of female body weight
• Sexual dimorphism:
– Females often larger (to produce more/larger eggs)
– Males sometimes larger when territorial during spawn (e.g.
salmon)
– Sometimes accompanied by changes in color (dichromatism)
and body structure during spawning season
Timing of Spawning
• Semelparity: Fish reproduce once, then die.
• Iteroparity: Fish reproduce repeatedly during adult
lifespan.
• Seasonal cycles:
– In temperate areas where seasonal fluctuations in climate are
significant, spawning usually happens as a discrete event (once
per year)
– In tropics, many fish species spawn year-round in either distinct
peaks or at a constant rate.
– Timing of spawning usually linked to corresponding conditions
for larval development and early growth.
Spawning Migrations
• Some fishes undergo directed movements to a
specific location for spawning
– In less dramatic instances, spawning may merely
involve a shift in habitat preference.
• Anadromy: Most growth takes place in salt
water, adult migrates into freshwater to spawn
(e.g. coastal salmonids)
– Lentic salmonids often migrate into feeder streams to spawn.
• Catadromy: Most growth in freshwater, migrate
to saltwater to spawn (e.g. anguillid eels)
Note more catodromous species in the tropics and the
reverse for temperate latitudes; why?
• Tropical oceans are extremely oligotrophic; yet rainforest
streams are rich.
• Temperate coastal oceans and higher latitude open ocean are
very productive relative to some streams.
Atlantic salmon (Salmo salar)
mountain mullet (Agonostomus monticola)
Overview of Tolerance Range versus
Survival, Growth, & Reproduction
Population Dynamics & Management
• Stock: total numbers of a population; sum of year classes.
• Production: biomass * growth rate of population.
• Age (size) classes: sub-groupings or cohorts within a
population.
• Recruitment: Numbers entering a new year class; can be
defined for each year class. Can have inter-annual variability.
• Mortality: Loss between years.
• Fecundity: Offspring numbers per female; may differ between
mature classes.
Age and Growth
• The patterns of abundance at age gives an indication of
the annual mortality of the population; the number of
individuals in each age class will decline at a rate
dictated by the total annual mortality.
– Therefore, characterizing the age structure of the population is a
central component in studying fish population dynamics.
– Age structure can be monitored to determine impacts of
exploitation / environmental perturbation on population
Determining Age of Fishes:
1. Length frequency analysis
2. Estimation from analysis of “hard parts” (scales,
otoliths, etc).
Length Frequency Analysis
• A comprehensive sampling of fish in the system is
required for complete age analysis; bias in the sample
leads to poor characterization of ages and lifespan.
Percent Frequency
• Lengths of all fish in sample measured, frequencies of each length
(i.e. number of fish present of a given length) plotted, and distinct
“groups” of length distributions are identified:
Analysis of Hard Structures
• Seasonal variation in growth rates creates distinct “marks” in radial
expansion of hard structures such as scales, otoliths (ear bones),
spines, etc.
• Similar to rings on trees, these marks can be used to count the
number of days or annual cycles the fish has experienced.
• Distinct changes in growth rate associated with seasonal variation in
temperature, seasonal variation in resource availability, energetic
losses due to reproduction / spawning, etc.
• Growth can be verified by chemical “marking” of hard parts
(OxyTertraCycline, etc.) and rearing / recapture of fish.
Recruitment
• Recruitment is a function of spawning stock size, densitydependent forces, and physical influences on mortality
and survival.
– Increasing numbers of mature classes produces large number of
offspring up to a limit.
– Too many mature individuals leads to competition for limited
resources, meaning more energy goes toward competing and
growth; less toward reproduction -> lower number of offspring.
– Large number of offspring compete for limited food supply,
leading to slower growth and increased mortality under crowded
conditions.
– Physical and disease conditions play an unpredictable role.
• Environmental influences on spawning and juvenile
recruitment:
– Spawning habitat; including physical and chemical conditions.
– Timing of hatching and larval development with prey resources.
Fisheries
• Commercial fisheries: Large numbers of fish harvested for sale,
generally on significant scales of exploitation. Fisheries hold high
importance both as food source and as source of income.
• Sport / Recreational Fisheries: Fewer numbers of fish harvested per
individual; however, much greater numbers of individuals
participating in fishery. Very difficult to assess impact of these
fisheries due to cryptic catches and mortality.
• Sustenance Fisheries: Can be small-scale commercial fishing
operations limited to local sale, or consistent harvest from a fishery
for purposes of nutrition / sustenance.
Fishery Data
• Historical catch data from fisheries provide indications on stock
status compared to previous (especially unexploited) levels.
• Trends in historic data can be difficult to interpret due to changing
abilities of fisheries (technological advancements, improved
techniques, etc.)
• Number / biomass of fish caught (catch) is not an unbiased means
of estimating of stock size; instead, must consider catch per unit
effort (hooks deployed, hours fished, # net sets, etc.) or yield.
• This requires understanding efficiency of different fishing
approaches and standardizing them against to a single unit of effort.
Response to Exploitation
• Under the simplest assumptions,
exploitation (harvest) of a stock functions
as an increase in mortality. We would
expect a compensatory response of
increased production due to decreased
density and competition.
• (see Maximum Sustainable Yield
approach).
Managing Fisheries: MSY
• The concept of Maximum Sustainable Yield is based
upon the principles of logistic growth and density
dependent net production.
dN / dt
300
250
200
150
100
50
0
0
100
200
300
400
500
N
At an intermediate stock size, the rate of net production (growth
not offset by mortality) is highest.
If this net production is harvested, the stock will continue to
produce this highest level of net “surplus”
MSY is catching fish in excess of the number needed to maximize
production of reproductive adults.
MSY and MEY
• Maximum economic yield (MEY) is different from MSY.
• Assuming a proportional relationship between effort and cost, we
can plot cost of effort versus yield on the same graph:
MSY
Yield and Cost
($Value)
600
500
MEY
400
Break-even point
300
200
100
0
0
50
100
150
Fishing effort (f)
200
But it’s NOT that simple:
– Stock is made up of fish of different ages, sizes,
maturity levels, reproductive capacity (i.e. fecundity).
– Fishing often targets largest & easiest-to-catch fish
foremost, leaving smaller, less fecund and sexually
immature fish.
– This can at best reduce the reproductive capacity of
the stock (spawning stock biomass as indicator of
stock health).
– At worst, this can in time change the biological
characteristics of the stock (size at maturity, growth
rates, etc.).
Methods for Managing Fisheries
• Controlling effort:
– By limiting or allocating effort, stock can be kept at sustainable
levels.
– Difficulties in enforcement, sources of cryptic mortality (hooking
mortality, etc.).
– Example: bag limits, gear restrictions, etc.
• Managing Production:
– Size restrictions are designed to increase reproductive potential
of unexploited stock.
– Minimum size of capture set above size (age) of maturity.
– Slot limits allow removal of medium sized fish, keep large (highly
fecund) fish in stock.
• Protecting vulnerable stages:
– Spawning aggregations
– Easily exploitable stages
– Example: closed seasons, closed areas
• Providing refuge: Protected Areas
– Allocates an area closed to harvest, provides spatial
refuge for stock.
– Can provide a consistent source of spawning stock,
dispersal extends recruitment to surrounding /
outlying areas.
– Currently at the forefront of fishery management.
Stock Enhancement
• Fish are artificially reared for release into
natural systems to augment natural
reproduction.
– Upside: consistent recruitment managed to
offset harvest.
– Downside: dangers to genetic makeup of wild
stock, deficiencies of hatchery fish.
– Stock enhancement vs. “Put-and-Take”
Trout stocked in PA!
Brook Trout Salvelinus fontinalis
Brown Trout Salmo trutta
Rainbow Trout Oncorhynchus mykiss
Aquaculture:
• Has potential to relieve pressure on wild stocks if net profit can
exceed that of commercial fishing.
• Becoming more widespread and including more prominent fish
species (salmon, shrimp, seabass) due to improving technology.