Biodiversity of Fishes Stock-Recruitment Relationships Rainer Froese 02.02.2017

Definitions A stock is a population of aquatic animals (mostly fish) that is exploited (fished) by humans (fishers) The stock size is referred to as biomass and is measured in tonnes Reproductively active fish (spawners) constitute the spawning stock Recruits are young fish that have survived and are about to join the spawning stock (measured in numbers, e.g. millions)

Typical S-R Data Recruits (N) Spawning stock biomass (tonnes)

The Hump (Ricker, 1954) Assumptions: where A = ln Rmax Assumptions: a) negative S-R relationship at high S b) highest recruitment at intermediate S

The Asymptote (Beverton & Holt 1957) where A = ln Rmax Assumption: Positive S-R relationship throughout

The Hockey-Stick (Barrowman & Myers 2000) Assumptions: Constant R/S at low S Constant R at high S

The Smooth Hockey-Stick (Froese 2008) where A = ln Rmax Assumptions: Practically constant R at high S Gradually increasing R/S at lower S

Example Striped bass Morone saxatilis Model α low up Rmax r2 B&H 3.67 2.60 4.73 24.9 17.3 36.0 0.834 Froese 3.40 2.64 4.15 17.4 13.5 22.6 0.843 Ricker 3.22 3.81 19.8 16.5 23.9 0.846 Parameters and accounted variance not significantly different Extrapolation VERY different

What Determines Recruitment? Eggs need to encounter right salinity, temperature, current, wind, and absence of predators After absorption of the yolk sac, larvae need to find food within days or reach a “point of no return” Juveniles grow up in small nursery areas with limited carrying-capacity (shelter & food) As a result of mostly starvation and predation, the number of recruits that a population can produce in a given ecosystem is limited Because of this limited recruitment, populations can not grow much larger than 4 times the population size needed to produce the minimum number of eggs needed for the maximum number of recruits

Use of Hockey-Stick in Management Conceptual drawing of the hockey stick relationship between spawning stock size and recruitment. SSBlim marks the border below which recruitment declines, SSBpa marks a precautionary distance to SSBlim, and 2 * SSBpa can be used as a proxy for SSBmsy, the stock size that can produce the maximum sustainable catch [ContHS.xlsx]. (Froese et al. 2014. Revisiting safe biological limits in fisheries)

How to Fit a Hockey-Stick Fitting a rule-based hockey stick: (1) calculate geometric mean of recruits in upper half of biomass range: gmean R = exp(average(log(R)) for R at SSB > 383 = 373

How to Fit a Hockey-Stick Blim Fitting a rule-based hockey stick: (2) Extend shaft to lowest biomass with same or higher recruitment. This gives Blim.

How to Fit a Hockey-Stick Blim Bpa Fitting a rule-based hockey stick: (3) Multiply Blim = 184 with 1.4 to get a precautionary Bpa = 258 .

How to Fit a Hockey-Stick Blim Bpa Fitting a rule-based hockey stick: (4) Connect Blim to origin, check fit with low recruitment.

How to Fit a Hockey-Stick Blim Bpa Bmsy Fitting a rule-based hockey stick: (5) Use 2 * Bpa = 516 as proxy for Bmsy.

52% of the stocks were outside of safe biological limits, and 12% were 2016 Abstract The appropriateness of three official fisheries management reference points used in the north-east Atlantic was investigated: (i) the smallest stock size that is still within safe biological limits (SSBpa), (ii) the maximum sustainable rate of exploitation (Fmsy) and (iii) the age at first capture. As for (i), in 45% of the examined stocks, the official value for SSBpa was below the consensus estimates determined from three different methods. With respect to (ii), the official estimates of Fmsy exceeded natural mortality M in 76% of the stocks, although M is widely regarded as natural upper limit for Fmsy. And regarding (iii), the age at first capture was below the age at maturity in 74% of the stocks. No official estimates of the stock size (SSBmsy) that can produce the maximum sustainable yield (MSY) are available for the north-east Atlantic. An analysis of stocks from other areas confirmed that twice SSBpa provides a reasonable preliminary estimate. Comparing stock sizes in 2013 against this proxy showed that 88% were below the level that can produce MSY. Also, 52% of the stocks were outside of safe biological limits, and 12% were severely depleted. Fishing mortality in 2013 exceeded natural mortality in 73% of the stocks, including those that were severely depleted. These results point to the urgent need to re-assess fisheries reference points in the north-east Atlantic and to implement the regulations of the new European Common Fisheries Policy regarding sustainable fishing pressure, healthy stock sizes and adult age/size at first capture. 52% of the stocks were outside of safe biological limits, and 12% were severely depleted.

Exercises Go to www.ices.dk, Community, Advisory process, Latest advice, Advice 2014; Select a stock with a stock-recruitment graph, e.g.: Western Baltic cod (cod-2224) Western Baltic herring (her-3a22) North Sea cod (cod-347d) Fit hockey-stick to stock-recruitment plot Discuss Blim, Bpa and Bmsy compared to official estimates What is the status of the stock?