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**Fishery selection and its relevance to stock assessment and fishery management.**

David Sampson Professor of Fisheries OSU Hatfield Marine Science Center Coastal Oregon Marine Experiment Station

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**Two Years in Northern Italy**

With the European Commission’s Joint Research Center in Ispra, north of Milan. The JRC provides research based scientific advice to support a wide range of European Union policies. Institute for the Protection and Security of the Citizen. Applied research & development aimed at analyzing, modeling and developing new security applications. Maritime Affairs Unit. Shipping container traffic; vessel surveillance & port security; scientific support to fisheries. FISHREG Action. ~ 25 fisheries scientists. The overall objective of the Joint Research Centre (JRC) is to help create a safer, cleaner, healthier and more competitive Europe. The JRC coordinates and contributes to numerous EU-wide networks linking industry, universities and Member State institutes, as well as carrying out studies and experiments in its own laboratories on behalf of its customers and stakeholders. The JRC’s work embraces a broad spectrum of tasks: from establishing standards for healthcare product approval to identifying the sources of illicit nuclear materials; from improving the earthquake-resistance of buildings to detecting the presence of genetically modified (GM) ingredients in foodstuffs; from assessing the quality and sustainability of water resources, to satellite monitoring of land use and deforestation. JRC research Institutes are situated in Ispra (Italy), Karlsruhe (Germany), Petten (the Netherlands), Geel (Belgium) and Seville (Spain). The other JRC institutes located at the Ispra site were the Institute for Environment and Sustainability and the Institute for Energy and Transport.

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At the JRC ... ... while building a bioeconomic simulator, David stumbled upon some surprising behavior related to fishery selectivity. Resulting publications: Sampson, D.B. and Scott, R.D A spatial model for fishery age-selection at the population level. Canadian Journal of Fisheries & Aquatic Sciences 68: Scott, R.D. and Sampson, D.B The sensitivity of long-term yield targets to changes in fishery age-selectivity. Marine Policy 35: Sampson, D.B. and Scott, R.D An exploration of the shapes and stability of population-selection curves. Fish and Fisheries (available on line). One extra click needed for an animation. At the JRC David had very general job duties: Provide advice in the field of fisheries research; Contribute to proposals for collaborative research; Contribute to the coordination of the Scientific, Technical and Economic Committee for Fisheries (STECF); Develop and test computer programs for simulating fishery systems (a spatial bioeconomic simulator, written in R). David’s co-author on the three selection papers was Rob Scott, a stock assessment scientist the CEFAS laboratory at Lowestoft, United Kingdom, who was working at FISHREG while on a leave of absence.

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**Talk Outline What is fishery selectivity?**

Issues related to gear-selectivity. Selection curve shapes and stability. A spatial model for fishery age- selectivity. Conditions that generate domed population-selectivity. Selectivity and MSY reference points.

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**What is fishery selectivity ?**

Part 1. What is fishery selectivity ? Arona

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**Fish abundance and catch-at-age Fishing mortality-at-age**

? What is Selectivity ? Fish abundance and catch-at-age Fishing mortality-at-age Young fish escape the gear or live elsewhere. Selection is F-at-age scaled so the maximum value is 100%.

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**Factors Influencing Selectivity:**

Gear selection. Fish age / size / behavior affect which fish are caught and retained by any type of fishing gear. The mixture of fishing gears. When there are multiple gear-types with differing gear- selection traits, the relative catches by each gear-type determine the population-level selectivity. Spatial locations of the fish and the fishing. Fishing gear operates at a local scale and can only catch fish that are near the gear. The population- level Cage depends on the spatial distribution of fishing operations relative to the spatial distribution of the fish.

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**Selectivity Factors: Gear Selection**

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**Selectivity Factors: Gear Mixtures**

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**Selectivity Factors: Spatial Effects**

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**Issues related to gear-selection.**

Varese Part 2. Issues related to gear-selection.

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**Selection by the Fishing Gear ? Age-Based or Length-Based ?**

If selection is by age, then no effect on observed length-at-age. Not so if selection is by length.

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**? What’s Wrong with these Graphs ?**

Assessment of Lingcod (Ophiodon elongatus) Are such changes in selection plausible?

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**Another Strange Selection Curve**

Assessment of Longspine Thornyhead (Sebastolobus altivelis)- 2005

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**Selectivity Propositions**

Fish that are about the same age or size should have the same relative vulnerability (i.e., selection). If estimates of selection by age (or by size) show abrupt changes between adjacent age-classes, there is probably something wrong with the model specifications.

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**Selection Curve Shapes and Stability: An Empirical Analysis.**

Part 3. Selection Curve Shapes and Stability: An Empirical Analysis. Gelati

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**Selection Curve Shape and Stability**

Virtual Population Analysis (VPA). Complete catch-at-age data to reconstruct abundance-at-age and F-at-age. No assumptions about selectivity. Widely used on both sides of the North Atlantic. F-at-age estimates from 15 published, peer-reviewed stock VPA assessments. F-at-age converted to smoothed selectivity curves estimates using GAMs. Test for Age, Year, and Age x Year effects.

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**Selection Shape and Stability (cont.)**

Increasing selectivity: American plaice on the Grand Bank Asymptotic selectivity: Atlantic cod on Georges Bank A selection curve was categorized as increasing if (1) the selection coefficients for each age-class were strictly increasing with age and (2) the condition for asymptotic selection was not satisfied. A selection curve was categorized as asymptotic if the selection coefficients for the three oldest age-classes were all ≥ 0.95.

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**Selection Shape and Stability (cont.)**

Domed selectivity: Atlantic herring (fall spawners) in the southern Gulf of St Lawrence Saddle selectivity: Atlantic herring in the Gulf of Maine and Georges Bank A selection curve was categorized as domed if (1) the age for peak selection was interior to the range of age-classes and (2) the asymptotic and saddle conditions were not satisfied. A selection curve was categorized as a saddle if (1) there was at least one local minimum selection coefficient interior to the range of age-classes and (2) the asymptotic condition was not satisfied.

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**fishery age-selectivity.**

Part 4. A spatial model for fishery age-selectivity.

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**A Mathematical Model for Selectivity**

Abundance-at-age (a) by region (i): M is the instantaneous rate of natural mortality. Fi is the instantaneous rate of fishing mortality in region i. Coefficient sa is the gear selectivity for age-a fish. Coefficient Pi,j is the proportion of fish that move into region i from region j at the end of each year. Survival of fish that stay in region i Survival of fish that migrate into region i The model for population-selectivity is derived from a set of coupled equations describing survival in a set of distinct spatial regions. Na,i denotes the number of live age-a fish in region i at the start of a year. M is the instantaneous rate of natural mortality. Fi is the instantaneous rate of fishing mortality in region i. The coefficient sa is the gear selectivity for age-a fish. The coefficient Pi,j is the proportion of fish that move into region i from region j at the end of each year.

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**Selectivity Model (continued)**

Abundance-at-age: F-at-age: Pop. selection-at-age: N.B. This is a cohort (equilibrium) model.

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**Now we will explore an Excel version of the population- selectivity model.**

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**Heuristic Explanation for the Dome**

Consider a stock in two regions, no movement between regions, same logistic gear selection curve in both regions. Higher F fewer old fish Lower F more old fish When David was building his bioeconomic simulator he needed to determine the population-level selectivity that resulted when fishing was spatially distributed. He kept getting dome-shaped population-selection curves, even though regional gear-selection was asymptotic. This result was entirely unexpected. At first he thought he had made some silly mistake in the R-code. Soon, however, he realized that the unusual behavior followed directly from the mathematics. Pictured on this slide are the F-at-age curves for a simple population model having two regions. Both regions have the same asymptotic gear-selection curve. The fishing mortality in one region is four times the fishing mortality in the other region. The F-at-age curves in each region are just scaled versions of the underlying gear-selectivity curve. The population-level F-at-age curve is an average of the two region-level F-at-age curves. The higher F-at-age curve generates larger catches of young fish than the lower F-at-age curve, but it leaves fewer old fish. The population selection curve is the average of the two Fage curves. The Region 1 curve (F = 0.4) dominates at young ages; the Region 2 curve (F = 0.1) dominates at old ages.

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**Conditions that generate domed population-selectivity.**

Part 5. Conditions that generate domed population-selectivity.

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**Conditions for Domed Selectivity**

Domed selection if Sa+1 < Sa. Under what conditions? The general conditions are difficult to discern because the equation for population selection is complicated. The gear-selection coefficients are embedded in the age- and region-specific total mortality coefficients, Za,i = M + Fi ∙ sa .

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**Domed Selectivity (continued)**

Condition for domed selection can be written Exponentiate and rearrange to get Direct substitution of the general equations for Na , Na+1 and Na+2 into this inequality produces a mess. But, useful results can be obtained from the simpler problem of no movement and constant gear-selection.

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**Domed Selectivity (continued)**

Consider the case of two regions. Decreasing selectivity implies

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**Domed Selectivity (continued)**

Similar reasoning leads to the solution for any number of regions. Population-selectivity (given no movement and constant gear-selection) will be decreasing if

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**Selectivity & MSY reference points.**

Venizia Genoa Part 6. Selectivity & MSY reference points.

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**N.B. No plus-group. All fish are dead by age A+1.**

Selectivity and MSY Equilibrium yield is derived from standard equations for yield-per-recruit, spawning biomass-per-recruit, and a Beverton & Holt stock-recruit relationship. Yield-per-recruit: Spawning biomass-per-recruit: N.B. No plus-group. All fish are dead by age A+1.

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**Selectivity and MSY (continued)**

B&H stock-recruit relationship: At equilibrium each recruit exactly reproduces the spawning biomass of its parents. Equilibrium Yield:

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**Selectivity and MSY (continued)**

Maturity Age50% Maturity Age50% M = 0.2; k = 0.15 Fishery selection influences DB-SRA results.

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Now we will explore an Excel version of the spatial population- selectivity model, extended to include the MSY calculation.

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**Summary and Conclusions**

Near Como Summary and Conclusions Sunrise from my bedroom

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**Summary of the Lessons Learned**

VPA results indicate considerable variation in population-selection. We should not be surprised to find that population-selectivity varies through time. (Constant selection is unusual.) We should not be surprised to find that population-selectivity is dome-shaped. MSY and related biological reference points are functions of selectivity and also the spatial distribution of fishing. A slide that I should have concluded my talk with.

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**Grazie per l’Attenzione**

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