Modelling the effect of marine protected areas on the population of skipjack tuna in the Indian Ocean

Abstract

-The benefits of implementing no-take Marine Protected Areas (MPAs) for the conservation of highly migratory species are not easy to assess. They depend on several factors, such as the fish mobility, fisher behaviour and the area covered by the MPA with respect to the distribution area of the species to protect. In this study, we explore the simultaneous effects of MPAs and fishing scenarios on skipjack tuna population dynamics, using the spatially-explicit APECOSM-E model. The model represents the size-structured population dynamics of skipjack tuna in the Indian Ocean and their dependence on climatic variability and exploitation by fisheries. Numerical experiments were run from the beginning of industrial fisheries in the early 1980s to the year 2030, considering different scenarios for the future development of fisheries. These scenarios combined different trends in fishing effort and technological development, either assuming a continuous increase following historical trends or a stabilization of these factors at present values. The simulations were designed to explore the effects of two MPAs of different size and location: the recently established Chagos MPA, and a hypothetical MPA covering a large part of the Western Indian Ocean, where most of the skipjack catches are presently made. We modelled the redistribution of fishing effort around the MPAs assuming that the fishers had partial knowledge of the spatial distribution of the skipjack population. The effects of the two MPAs on the population dynamics, catch and fishing mortality are shown. Our results revealed a very minor effect of the Chagos MPA on the skipjack tuna population, while the Western Indian Ocean MPA had an important impact on the fishing mortality and succeeded in stabilizing the spawning population. The simulations also showed that the effect of an MPA depends on the evolution of fisheries and it is therefore important to explore different fishery scenarios to assess the future benefits of an MPA.

Minimising unsustainable yield: Ten failing European fisheries

“…However, whilst the enshrinement in EU law of the imperative to attain MSY is undoubtedly welcome, the use of a traditional fisheries management paradigm of sustainability has been shown to be ineffective in these fisheries. Reasons for this include political intrusion [11,29,36], delayed management response [17,23,56], non-consideration of climatic and prey species variation in stock estimates [31,42,47,50,71,75], systemic incentivisation of discarding [59,68,75] and unforeseen interactions between exploited stocks [8,76,77].…”

“…Any measure proposed must balance effectiveness with feasibility, but problematically, T. trachurus has been shown to exhibit indeterminate annual fecundity, meaning that a single annual measure will not be sufficiently accurate and that multiple surveys per annum must be completed [58]. Given that the effectiveness of MPAs in protecting migratory scombrid or carangid species is currently poorly understood and largely untested [59], spatial protection represents less of a priority for this stock than the evolution of appropriate SSB measurement devices.…”

Modelling the impacts of marine protected areas for mobile exploited fish populations and their fisheries: what we recently learnt and where we should be going

“…By contrast, the Western Indian Ocean MPA had a considerable effect on the fishing mortality of K. pelamis and succeeded in stabilising the SSB of the species, but resulted in decreases in fisheries yields. Although a loss in fisheries yields occurred over the period 2010-2030 with the Western Indian Ocean MPA, declines in fisheries yields over the period 2010-2030 were more pronounced in the absence of MPAs (Dueri and Maury 2013).…”

“…In recent years, for the first time in the history of MPA modelling, two studies investigated the conservation and fisheries effects of MPAs for highly migratory fish populations (Dueri and Maury 2013;Sibert et al 2012). …”

“…Empirical evidence suggests that the K. pelamis population of the Indian Ocean is overexploited at present (IOTC 2010). Dueri and Maury (2013) ran the APECOSM-E model from the early 1980s to the year 2030 under alternative MPA scenarios (starting from 2010). The effects of two different year-round, no-take MPAs were examined: the existing Chagos Island MPA (Koldewey et al 2010); and a hypothetical Western Indian Ocean MPA covering a substantial amount of the area where habitat is favourable to K. pelamis and the species is currently exploited (around one third of the Western Indian Ocean).…”