Recruitment = reproduction = fecundity = natality • Fecundity = the # of ripening eggs per female prior to the next spawning season • Natality = the rate of addition of new individuals to a population through reproduction • Recruitment = the number of individuals still alive at any specified time stage after the egg stage – larvae, fry, juveniles
Why is the relationships between stock and recruitment important to fisheries management? • Yield - depends on number of recruits
• Spawning stock can be controlled through management
Recruits
Why is the relationships between stock and recruitment important to fisheries management?
Spawning Stock
Three ages of recruitment • Recruitment to the stock - at age tr – age at which fish are recruited to the fishery; moment or interval during which it becomes in some degree vulnerable to capture by the fishing gear in use – can be determined by size, habitat, characteristics of gear
• Recruitment to exploited stock - at age tc – age at first capture. – controlled through regulations; legally exploitable – tr ≤ tc
• Recruitment to spawning stock - at age ts
Types of recruitment • Knife edge - instantaneous - all fish of a given age become recruited at the same time • Platoon - in a give year only a fraction of the youngest age are fully catchable – year class is divided into two platoons recruited and non-recruited
• Continuous - there is a gradual increase in vulnerability of members of a year-class – most common
Stock - recruitment models • Normally consists of looking at the empirical relationship between the spawning stock size, and the subsequent recruitment of the year class
Recruits
4000
0
0
Spawning Pop. Size 1600
General modeling approach Eggs Larvae Juveniles Recruits
• Fish must pass through 3 or more distinct life history stages before recruiting • At each stage, survival is dependent on competition, predation, and environmental influences
Theory Behind StockRecruitment Models • Population sizes are stabilized by negative feedbacks between stock size and recruitment • Biological processes that will affect stock and recruitment – density independence – density dependence - mortality is compensatory • higher mortality occurs at higher densities • want to harvest some fish to cause better survival in some individuals
Recruits
Spawning Stock
Common Stock - Recruitment Models 1) Ricker Model - 1954 2) Beverton - Holt Model - 1957
Ricker stock - recruitment curve 2500
R =α ⋅Pe
Recruits
2000
− βP
1500 1000 500 0 0
500
1000
1500
Spawning Stock
2000
2500
Assumptions for the Ricker Model 1) mortality rate of the eggs and juveniles is proportional to the initial cohort size
Mechanism leading to a Ricker shaped recruitment curve 1) cannibalism of the juveniles by the adults 2) disease transmission 3) damage by adults of one anothers spawning sites 4) density dependent growth coupled with size dependent predation
Use of the Ricker curve 1) Set quotas for catch of fixed escapement policy 2) Predict future population abundance cycles inherent in curve - For single age spawners - slope of descending limb determines cycles 1) slope = -1 permanent oscillations of equal magnitude 2) Slope between 0 and -1 - dampened oscillations 3) Slope < -1 = permanent cycles
Beverton - Holt Model 1 R= α+β P
1800 1600 1400
Recruits
1200 1000 800 600 400 200 0 0
500
1000
1500
Spawning Stock
2000
2500
Assumptions of the Beverton Holt Model 1) juvenile competition results in a mortality rate that is linearly dependent upon the number of fish alive in the cohort at any time
Ecological difference between Ricker and Beverton-Holt model • Ricker assumed that density-dependence was based on mobile, aggregating predators • Beverton-Holt assumed that predators were always present
Which to use? • Which makes more sense based on principles?
Problems with both models • Do not account for numbers of spawners becoming so low that compensatory mechanisms can no longer occur – Allee effect - inability to find mates (or low fertilization success) with low broodstock numbers – Predation occurs where the number of prey eaten by predators is relatively constant. If spawning stock becomes very small. They may not be able to produce enough recruits to recover to previous levels
Important considerations • Process and patterns of recruitment • When a model is wrong, it usually leads you to believe that recruitment will not decline. This leads to overexploitation. • It is difficult to detect underlying stockrecruitment relations in a variable environment • Need a broad range of spawning stock sizes to see density dependence
Conclusions • Recruitment is highly variable • Two common density dependent models ( Ricker and Beverton - Holt) are inadequate for prediction and management alone • Forecasting of recruitment is improved by developing pre-recruit monitoring programs
