Is fishing during the breeding season incompatible with the renewal of stocks?
This section aims to provide objective answers to various questions that may arise about the resource and the fishing profession. For the second episode of "Fishing in questions", Alain Biseau, project manager at Ifremer, coordinator of fisheries expertise (Department of Biological Resources and Environment) and member of the ICES advisory committee agreed to answer our questions on the renewal of fish populations.
Q What are the conditions that ensure the renewal of a population?
Renewal of a population is assured when births compensate for deaths. A population is in equilibrium when the small fish of reproduction come to replace the fish, caught or dead in a natural way (predation, old age ...).
Ensuring the renewal, the balance, of a population is essential to the sustainability of fishing; the size of the population must be compatible with management objectives and economic profitability. Obviously, the smaller the population, the greater the risk of collapse of the resource and the fishery (which does not mean extinction of the species). There is therefore a minimum threshold of biomass below which we must not fall: it is the precautionary approach. The current management objectives that target the RMD imply keeping the populations above this minimum threshold.
Q What are the factors that can have an effect (positive / negative) on the renewal of a population and in what way?
The factors that affect the dynamics of a population and therefore its renewal are numerous. Fishing is one of many; environmental factors such as the quality of water, habitats and the ecosystem in general (including availability of food, presence or absence of predators, wind direction and currents transporting eggs and larvae) are equally important.
Fishing is often the adjustment variable because it is easier to control than the other environmental factors conditioning renewal. There are few possible human actions to positively influence the renewal of a population (difficult to consider putting ice in the sea to cool it as noted (with good reason!) A former president of the CNPMEM) except limit negative impacts (reduction of fishing effort, spatio-temporal closure). In some particular cases, however, the establishment of artificial reefs may help to improve population renewal, either by limiting fishing or by providing habitats favorable to the development of eggs and larvae.
Q What is the relationship between spawning biomass and the level of recruitment?
To have babies, you need parents. No breeding, no new births and therefore no recruitment, this is the only certainty. But, beyond a certain amount, an increase in the number of breeders does not systematically lead to an increase in recruits, when certain environmental conditions induce high mortality of the young stages. On the contrary, it has often been observed that when the number of breeders is lower, the survival rate of eggs and larvae is better. There is therefore no direct proportionality between breeding biomass and recruitment beyond a certain threshold.
Below this threshold, the risk of collapse (very low level of recruitment and therefore no renewal of the stock) is high. This is called a precautionary threshold or biological safety limit.
Is there an optimum period (in relation to breeding) for fishing? In other words: when to fish? before, during or after the breeding season?
A fish caught before, during or after the breeding season is primarily a dead fish. Fished before, it will not contribute to the reproduction of the year, but fished after, it will not contribute to that of the following year ... From the strict point of view of the quantity of reproducers, it is advisable to frame / to limit the total annual withdrawals to ensure a sufficient quantity of breeders; the capture period does not matter.
Q Is spawning / spawning compatible with stock renewal and RMD operations?
Once the quantity of breeding stock is maintained at a sufficient level and the breeding process is not affected, spawning or breeding season fishing is not incompatible with the renewal of the stock and its exploitation at RMD.
Q Is spawning fishing a risk of collapse for fish stocks?
To qualify the previous answer, even if the spawning level is maintained at a sufficient level at the stock level, fishing on the spawning grounds may present two risks: the first is the disturbance by fishing (but this is also true for any other anthropogenic activity) of the reproduction process (dispersion of the benches, acoustic interference, etc.). The second is the possible local depletion (we capture all the fish of a spawning ground) which would lead to an impoverishment of the genetic diversity of the populations concerned and therefore of their potential of adaptation.
In short, fishing on the spawning grounds, when spawners congregate, can only be considered with the guarantee of maintaining a sufficient level of breeding stock and genetic diversity. It can not be conceived without strict supervision.
Q Are there critical operating thresholds at which managers need to adopt management measures specific to the breeding season?
When a resource is very over-exploited (very low number of breeders) all means must be deployed to ensure its recovery. The protection or restoration of areas of fishing interest (spawning grounds, but also nurseries) is one of the means. The measures taken in this case must then concern all human activities (fishing and others).
In addition, when several trades target a stock, some during the breeding season with large catches, others throughout the year with smaller catches, the necessary reduction in total catches is often easier to obtain (and generally impacts fewer vessels) by limiting fishing during the breeding season, and therefore the activity of a trade type.
Q What are the advantages and disadvantages of spawning?
There is a need to distinguish between spawning (area and time when fish are concentrated for reproduction) and fishing during the breeding season (without breeding concentration).
Fishing during the breeding season has no advantage or disadvantage (except those possibly related to the quality of the fish and / or its recovery) provided that the maintenance of a minimum quantity of breeders is ensured by an overall limitation of catches and / or or good selectivity. Regarding spawning, we have already mentioned the disadvantages (behavioral disturbance, risk of genetic impoverishment), but there may be advantages: for vessels targeting fish concentrations (especially during the breeding season) the quantity of fish caught per unit effort (and therefore per liter of diesel) is higher than that captured the rest of the year. However, the activity of these targeted fisheries on concentrations can strongly impact that of other trades, especially those who practice all year; not to mention recreational fishing.
So there are advantages and disadvantages. Fisheries management must reconcile the three pillars of sustainable development: environmental, economic and social. For sequential fisheries (catches of a stock made by several trades that succeed one another over time) a sharing of fishing opportunities between all users (or even a sharing of space) seems essential. This sharing is not biology but a political decision.
Q Is the share of EU-level breeding catches compared to total catches known?
Not really. However, since the catches are made, most of the year, those caught during the breeding season constitute a significant proportion of the total catches.
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