Transcript Fish & Fish Productivity - Penn State York Home Page

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
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Broadcast spawning
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Meroplanktonic (not seen in exclusively lotic species)
Benthic spawning
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Brood hiding (trout)
Brood guarding (cichlids)
Nest spawning (bass, sticklebacks)
Internal bearers: internal fertilization.
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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.
• Production: growth rate of population.
• Age (size) classes: sub-groupings or cohorts within
a population.
• Recruitment: Numbers entering a new class; can
be defined for each class.
• Mortality: Loss between classes.
• 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 best 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).
• However, it is not always 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.