Impact of increasing deployment of artificial floating objects on the spatial distribution of social fish species

Abstract

Summary1. Approximately 300 pelagic fish species naturally aggregate around floating objects (FOBs) at the surface of the oceans. Currently, more than 50% of the world catch of tropical tuna comes from the industrial tuna fisheries around drifting FOBs. Greater understanding of the complex decision-making processes leading to this aggregation pattern and the impact of the massive release of artificial FOBs by fishermen on the spatial distribution and management of tuna is needed. 2. We analyse how the interplay between social (relationships between individuals) and non-social (responses to the environment) behaviours may affect the spatial distribution of a population in a multi-FOB environment. Taking the example of tropical tunas associating with FOBs and using differential equations and stochastic simulations, we examine how, when increasing the number of FOBs, fish aggregation dynamics and the distribution of the population among patches are affected by the population size, level of sociality and the natural retentive and/or attractive forces of FOBs on individual tuna. 3. Our model predicts that, depending on the species' level of sociality, fish will be scattered among FOBs or aggregated around a single FOB based on the number of FOBs deployed in a homogeneous oceanic region. 4. For social species, we demonstrated that the total fish catch is reduced with increasing FOBs number. Indeed, for each size of population, there are a number of FOBs minimizing the total population of fish associated with FOBs and another number of FOBs maximizing the total population of associated fish. 5. Synthesis and applications. In terms of fisheries management, the total catch volume is directly linked to the total number of floating objects (FOBs) for non-social species, and any limit on the number of sets would then result in a limit on the total catch. For social species (e.g. tuna), however, increasing the number of FOBs does not necessarily lead to an increase in the total catch, which is a non-intuitive result. Indeed, our model shows that, for specific values of the parameters, deploying a greater number of FOBs in the water (all other parameters being constant) does not necessarily help fishermen to catch more tuna, but does increase the level of fishing effort and bycatch.

Fine-scale 3-dimensional movement behaviour of silky sharks Carcharhinus falciformis associated with fish aggregating devices (FADs)

“…Understanding the details of the species' associative behaviour is a key step toward fulfilling this knowledge gap. This information can then be integrated into modelling approaches used to predict how changing FAD densities can affect populations, such as those recently described for tunas by Sempo et al (2013).…”

Population assessment of tropical tuna based on their associative behavior around floating objects

“…We considered a system of N fish individuals in an array of p FADs434445. A fish can be in one of the two following states: it can either be associated with one of the FADs, or be unassociated, i.e., occupy a portion of the sea outside of the zone of influence of any FAD.…”

“…The time evolution of the number of associated fish is described through a system of p differential equations of the form43:…”

“…Here, the following case studies were considered: (i) variable association dynamics μ / θ , (ii) population sizes N and (iii) numbers of FADs p . For case study (i), the departure probability θ was fixed to a constant value and only the dependence on the association probabilities μ was studied, since the properties of the homogeneous system at equilibrium only depend on the ratio μ / θ 43. Secondly, we studied a heterogeneous system with two different FAD classes, denoted as FAD-class A and B (with model parameters μ A , θ A and μ B , θ B respectively).…”

Second-guessing uncertainty: Scenario planning for management of the Indian Ocean tuna purse seine fishery

“…Moreover, as the use of FADs varies by region, this shift in fishing practice also results in a subtle adjustment in fishing grounds, with more effort allocated into western grounds where floating objects exist at higher densities. By the mid-2020s the localised deployment of more FADs in these grounds results in a decrease in the average size of sets and an increase in search costs, as vessels must move between a higher number of FADs to achieve sufficient catches (see [41]). This impacts on the profitability of fishing firms, especially those operating larger, less fuel efficient vessels, and some choose to tie up a number of vessels in their fleets temporarily until fishing conditions improve.…”