ICES CM 2019/P:590
Arni Magnusson, Jacob Kasper, Cecilia Pinto, Paris Vasilakopoulos, Alejandro Yáñez, Knut Korsbrekke, David Miller
Fisheries stock assessment and management advice mainly focuses on the overall fishing pressure and annual catches in tonnes. Much less attention is paid to selectivity, whether it would be advisable to shift the fishery towards younger or older fish. In this study, we look at the main Atlantic cod stocks in European and North American waters (n = 15) and evaluate the likely effects of shifting the selectivity towards younger or older fish. The analytical approach incorporates the life history, stock size, and average catch composition from each fishery in recent years. The majority of cod fished in the North Sea and Celtic Seas tend to be at age 1 and 2, while ages 5 and 6 are most common in the catches of Northeast Arctic and Icelandic cod, with the other cod stocks somewhere in between. The stocks that are fished at the youngest ages are also the stocks with the fastest somatic growth rate. Essentially all the Atlantic cod fisheries would benefit from shifting the selectivity to older fish, with a predicted increase in both the annual yield and spawning biomass in the long term. A similar analytical approach could be used to evaluate the likely effects of shifting the selectivity of other fish species sharing habitat with the cod.
Coordinators: Arni Magnusson, Jacob Kasper
Arni Magnusson, Jacob Kasper, Cecilia Pinto, Paris Vasilakopoulos, Alejandro Yanez, Knut Korsbrekke, David Miller
May join later: Daniel Howell, Michael Schirripa, Tim Earl
Looking across many stocks of Atlantic cod, we see very different selectivities: some are fished mainly at ages 1-3, other stocks mainly at ages 4-8. This has not been the result of scientific advice, but gear regulations that have evolved in each region.
Can this be considered mismanagement of some of these cod stocks, in the form of underutilizing or risking stock collapse? Or are the different selectivities quite sensible, given the growth and maturity ogives for the respective stocks?
To evaluate whether some Atlantic cod fisheries could be improved (in terms of long-term yield, sustainability, precautionary approach, etc.) by changing the selectivity. In other words, provide an analytical foundation that could lead to scientific advice to change gear regulations.
With relatively little amount of work, we can give an important and useful summary/comparison of many Atlantic cod fisheries. A follow-up study could repeat the analysis for different species found in mixed fisheries on a regional basis.
Research questions:
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What are the long-term impacts of a given selectivity pattern?
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What should selectivity be optimized for? (Yield, economics, sustainability, CPUE)
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Which fisheries are not using an optimal selectivity pattern and what are the impacts of these decisions?
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Does using too young of an age plus group mask important nuances of selectivity patterns?
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Gather data for as many Atlantic cod stocks as possible (by year and age):
- Weight
- Maturity
- Natural mortality
- Fishing mortality
- Population numbers
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Within-stock analysis:
- Calculate averages - weight, maturity, selectivity, recruitment
- Biomass of average cohort, in the absence of fishing (barplot)
- Identify optimal age of harvest (highest column in barplot)
- Compare with selectivity
- Calculate optimal Fbar for this stock, given constant recruitment
- Evaluate how selectivity could be changed to improve this cod fishery, by sliding the selectivity pattern towards younger or older fish
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Across-stocks analysis:
- Compare all stocks, using summary statistics and composite plots
| Cod stock | Data | CSB plots | Potential | Historical |
|---|---|---|---|---|
| (R functions) | Arni | Arni | Cecilia | |
| --- | - | *** | ||
| Faroe Plateau | Jacob | Jacob | Cecilia | |
| Georges Bank | Jacob | Jacob | ||
| Gulf of Maine | Jacob | Jacob | ||
| Iceland | Arni | Alejandro | ||
| NE Arctic | Jacob | Jacob | ||
| North Sea | Arni | Arni | ||
| W Baltic | Cecilia | Jacob |
CSB plots show C=catch at age, S=selectivity, and B=biomass of cohort in the absence of fishing
Potential yield is calculated using the applyFmax function, using a variety of selectivity curves with different A50 values
Historical analysis looks at long-term changes in weights, maturity, and selectivity
Specific topics
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Papers on Lopt, optimal length at capture (Paris)
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Papers where Norwegian fisheries managers ask for analysis/advice on selectivity (Daniel)
Links
- Brunel and Piet 2013 The current paradigm in fishery management consists of promoting selective fishing in order to protect the youngest fish to let them grow and have the opportunity to reproduce before being caught.
- Diekert, 2010 "the current exploitation pattern implies that essentially the wrong fish are targeted. Catching older and heavier fish could double the fishery’s Net-Present-Value."
- Jørgensen et al. 2009
- Caddy and Seijo 2002 argues to protect the largest (especially female) fish
- CAPAM
- CAPAM workshop report
- Francis 2017
- Froese et al. 2008 Paper on Lopt
- Froese et al. 2016a Looks at balanced harvesting of sizes, and proposes sustainable and low-impact fishing instead.
- Froese et al. 2016b Proposing new equations for Lopt and Aopt
- Garcia et al. 2012 argues balanced harvest
- Hixon et al. 2014 argues to protect the largest (especially female) fish
- Kindsvater et al. 2017
- Law et al. 2012 argues balanced harvest
- Vasilakopoulos et al. 2016 "Currently, MSY and FMSY are usually calculated keeping selectivity constant (ICES 2011). Our meta-analysis, in line with previous stock-specific studies (e.g. Froese et al. 2008; Scott and Sampson 2011; Colloca et al. 2013), suggests that this traditional calculation of MSY is very restrictive in that it ignores the higher MSYs that can be extracted by shifting selectivity towards older/larger fish."
- STECF 2015 Technical measures report. Page 47: Trade-offs between selectivity and F for hake. "Increasing size/age-at-selection allows greater exploitation rates and greater yields to be extracted at lower levels of stock depletion."
- Vasilakopoulos et al. 2014 "an optimal exploitation regime is more than a single value of F applied at an optimal selectivity level. Rather, there is a species-specific continuum of optimal combinations of F and selectivity corresponding to areas of high long-term yield and SSB."
- Scott and Sampson 2011 "The results illustrate the potential extent of variation in MSY and the corresponding fishing mortality required to achieve it (FMSY) that may result solely due to changes in population selectivity. They show that relatively subtle changes in selection can produce substantial differences in MSY and FMSY."
- Sampson and Scott 2012 An exploration of the shapes and stability of population–selection curves. "For all 15 stocks, the population–selection curves were not stable through time but underwent changes in shape, which in some cases were quite radical."
- Sampson 2014 Fishery selection (selectivity) measures relative fishing mortality-at-age. Fishing gear and the locations of the fish and the fishing influence selectivity. VPA estimates indicate diverse selectivity curve shapes and temporal variability. Selectivity influences fishery performance and stock assessments.
- O'Boyle et al. 2016 "Building simulations that reflect the life history of the stock can guide assessment efforts by placing priors and constraints on model fits to selectivity patterns and be used to explore trade-offs between model complexity and the ability to produce reasonable management advice. Their development is encouraged as a standard feature in the assessment of migratory fish stocks." Expands on Sampson's work.
- Vasilakopoulos et al. 2020
Less relevant for our study...
Preliminary plot, from Icelandic cod analysis
We will have a Skype call the second Thursday of every month until the next MGWG meeting. We will organize tasks to be completed prior to each meeting.
11 Oct 2018 4pm (CEST, UTC+2)
- Examine, summarize, and interpret the results so far
- Review gaps in data, identify stocks that could be added
- Status update on side stories (Lopt, Norwegian literature, historical changes)
- Prepare a list of next tasks, decide who will report on what next meeting
08 Nov 2018
13 Dec 2018
10 Jan 2019 4pm (CET, UTC+1)
- Overview of datasets, recently added stocks, summary plots
- Profile results from
applyFmaxandapplyF0.1across all stocks, shifting selectivity to younger/older fish, effects on catch and SSB - Summarize the results, by focusing on the effect of shifting one year up or down
- Comparison of maturity and selectivity
14 Feb 2019
14 Mar 2019 4pm (CET, UTC+1)
11 Apr 2019 4pm (CEST, UTC+2)
09 May 2019 4pm (CEST, UTC+2)
13 Jun 2019 4pm (CEST, UTC+2)
11 Jul 2019 4pm (CEST, UTC+2)
08 Aug 2019 4pm (CEST, UTC+2)
12 Sep 2019 4pm (CEST, UTC+2)