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.
16 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).
17 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.
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
Percent Frequency 18 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.
19 (No Transcript) 20 Recruitment
Recruitment is a function of spawning stock size, density-dependent 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.
21 (No Transcript) 22 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.
23 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.
24 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).
25 Managing Fisheries MSY
The concept of Maximum Sustainable Yield is based upon the principles of logistic growth and density dependent net production.
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. 26 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
27 But its 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.).
28 Methods for Managing Fisheries
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.
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
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.
30 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
31 Trout stocked in PA! Brook Trout Salvelinus fontinalis Brown Trout Salmo trutta Rainbow Trout Oncorhynchus mykiss 32 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.