Marine protected areas in 'nonlinear' ecosystems

Abstract

The very large changes observed within marine communities, owing to excessive harvesting, have been attributed to switches between alternative stable states. Correspondingly large reductions in overall fishing effort are usually difficult to implement. For such 'nonlinear' ecosystems, introducing large marine protected areas, with low to zero harvesting, but without reduction in overall fishing effort, can give a marked increase in total yield of the depleted stocks. These increases, however, are still less than can be achieved by reducing fishing effort.

Point-of-View: Arguments for conventional fisheries management and against no-take marine protected areas: only half of the story?

“…It is increasingly being argued that CFMAs, such as quotas, effort reduction, partial seasonal closures and technical measures, will often be more effective than NTMPAs for sustaining fish stock yields (Shipp 2003(Shipp , 2004Steele and Beet 2003), mainly because most fish stocks migrate over a wide geographic scale and are therefore inappropriate for protection through site-specific measures such as NTMPAs. Furthermore, Shipp (2003Shipp ( , 2004 and Grimes and Ralston (2003) argue that CFMAs have proved to be quite effective for the majority of stocks, stressing that only a few stocks are actually considered to be overfished.…”

“…Whilst these authors accept the CFMAs will always have a role, they are strongly arguing that it is essential that a significant proportion of the world's seas is also set aside as NTMPAs to sustain marine ecosystems, including their component fish populations. Hilborn et al (2004aHilborn et al ( , 2006, Kaiser (2004Kaiser ( , 2005, Shipp (2003Shipp ( , 2004 and Steele and Beet (2003), on the other hand, are primarily focussed on the potential role of NTMPAs in sustaining fish stocks. Their assessments do consider the impacts of fishing on habitats and non-target species, but their primary focus is on improving the potential of CFMAs to promote sustainable fish stock yields.…”

Bioenergetics, overcompensation, and the source–sink status of marine reserves

“…However, they assume that diffusion migration, i.e., migration of standing stock, results in a flow from high to low density and hence that the MPA is a source. The same is true for the models of Neubert (2003) and Steele and Beet (2003), which are equally based on the logistic growth model. The trophic models of Baskett (2006Baskett ( , 2007 and Baskett et al (2006Baskett et al ( , 2007 also assume diffusion migration and hence do not need to distinguish between net-or gross-production allocation.…”

“…By consequence, the allocation models assume different behaviour at low food conditions: in the gross-production model, individuals reproduce as long as the food concentration is positive, whereas in the net-production model, there is a threshold food concentration below which individuals stop reproducing (Gurney et al 1996). Simple MPA models formulated in terms of ordinary or partial differential equations (ODEs or PDEs, respectively) are often based on LotkaVolterra type models (Neubert 2003;Steele and Beet 2003;Baskett et al 2006), without explicitly considering bioenergetics. We show that such models are equivalent to an implicit assumption of gross-production allocation.…”

Evaluating the biological effectiveness of fully and partially protected marine areas

“…Despite the evidence of beneficial effects of no-take marine reserves for enhancing biomass and density of exploited species within the boundaries of the reserve [12][13][14], enhancing reproductive output [15,16], rehabilitating community structure [17,18] and biomass export through adult migration [19][20][21], the prohibition of all extractive activities in certain areas can have negative socio-economic impacts [22]. Indeed, even if reserves benefit fisheries, local fishers may be negatively affected by the loss of fishing grounds, decreased catches and increased travelling time to fishing grounds, at least in the short-term [23,24]. Consequently, no-take marine reserves may face strong opposition from fishers, making the process of reserve implementation and subsequent enforcement difficult.…”