Where have all the big and old fish gone?

About the project

Where have all the big and old fish gone?

This is a question being asked globally and has engaged scientists, managers, politicians, and media outlets all over the world. In reality, the answer to this question is likely to be complex; global warming reduces fish body size, fishing often removes larger fish, and evolution favours fish that reproduce at younger ages prior to removal. Here in Sweden, there are many stories linking a lack of recovery of Baltic Sea cod, or reductions in herring stocks, to a loss of the older and larger fish.

Does size and age matter?

In fish it really does. Older and larger fish are better at reproducing, they spawn more often and produce better quality eggs. This means they contribute disproportionally to the next generation and are critical to replenishment. Older and larger fish are also more resilient, they spawn earlier in the year and over a longer period, and can survive in harsher environments, meaning they might be better placed to adapt to future environmental change. They are also thought to be educators, passing information on migratory routes and favourable locations to younger fish. Despite all this, older and larger fish are rarely protected.

If we continue to remove the most important part of the population (the older and larger fish), we are likely to be left with fish stocks that are slower to recover from exploitation, are less resilient to environmental change and can, in the long term, only provide our growing human population with fewer and fewer fish. Consequently, as the scientific evidence supporting the importance of older and larger fish builds, surely it is time for a new approach.

Does fisheries advice not already incorporate the importance of size and age?

No, not really. Fisheries advice is typically based on a target rate of fishing mortality and a stock’s estimated size. This ignores size and age, and it is generally assumed that that all mature fish contribute equally to spawning. This could lead to advised catches that are systemically too high, especially if stocks do not have the age and/or size structure (i.e., enough older and larger fish) to recover from current and past exploitation. Part of the problem here is that scientists and managers currently lack the tools needed to incorporate size and age into fisheries advice. In particular, there is a need for:

(1) indicators that can robustly describe the size- and age-structure of a stock,
(2) reference values that provide meaningful thresholds for what is considered good, or bad, to help inform management action.

In this project, we will deliver both. Specifically, we will take a new indicator (created by us at SLU Aqua) and turn it into an operational tool for global fisheries management.

So what is this new indicator?

The indicator is an age-based indicator (ABI) called ABI_MSY. ABI_MSY stands for the age structure relative to the age structure estimated at Maximum Sustainable Yield (MSY). The benefit of ABI_MSY is that it uses the same machinery that is currently used to estimate stock status and provide advice globally. It also has an established reference point, namely the age structure we would expect to observe if fishing occurred at MSY in the long-term.

What’s the plan in this project?

ABI_MSY currently exists in a science paper and work is needed to make the jump from paper to policy. To do this, we plan to:

(1) increase biological realism in ABI_MSY by accounting for the inherent randomness of biological processes like recruitment;

(2) increase utility by turning our reference point into a distribution, which will better describe any uncertainty, and allow us to calculate the probability and risk associated with a given management action;

(3) increase generality by providing different reference levels that move beyond MSY and provide alternative options to decision makers;

(4) increase userability via the development of a free-to-use R package that will facilitate the global use of ABI_MSY.

This work will provide scientists and managers with the tools they need to make age structure a focal point for management action and fisheries advice.

– The age- and size-structure of fish stocks is a very hot topic right now. For too long, we’ve been giving advice on fishing based on stock size (how many fish there are) and fishing pressure (how many fish we catch), without asking whether a stock has the age- and size-structure it might need to sustain or recover from that level of catch. By addressing this need, I firmly believe that our work will help advance and inform the sustainability of commercial fisheries both here in Europe and across the globe, Chris concludes.

Christopher Griffiths