According to the United Nations Food and Agriculture Organization, a full 22 percent of world fish stocks were overfished as of the late 1990s; that number is now thought to be closer to one-third of the global total.  These unassuming figures couch the hard fact that the continued persistence, the very existence, of a significant fraction of the myriad marine species on our planet is now in jeopardy due almost solely to the massive scale on which we have pulled, and in many cases are continuing to pull, them from the water.  The majority of those stocks not yet overfished are currently classified as “fully exploited,” signifying that there is little room for error by population biologists and fisheries managers before these populations, too, teeter over an invisible and uncertain threshold toward population decline.  And it is not just biodiversity at stake in these crashes—no fish means no fisheries, and no fisheries means no more fishermen, no more fishing communities (and no more fishing lobby, Washington).

It is in this context that Dr. Louis Botsford’s research on marine protected areas (MPAs) must be placed.  Conventional fishery management—that is, command-and-control limitations on fishing effort, gear, location, and duration—has been fairly unsuccessful in maintaining or regenerating healthy populations; Botsford quoted a mere three percent of stocks as “recovering.”  This failure is due in part to the difficulties in collecting comprehensive data on marine species and the correspondingly huge uncertainties involved in modeling fished population trajectories and setting appropriate management goals, a dilemma cleverly illustrated by Botsford’s “Weird Relative’s Will” model, which envisions global fish stocks as a public bank account.  Fisheries managers must determine how much to withdraw (or allow to be withdrawn) from this account each year without complete knowledge of the principle in the account (fish species abundances) or the interest rate at play (fecundity and recruitment), much less the complicated interrelationships between the two or the consequences of extraction thereon.

As a result, managers have begun to apply the MPA framework—which has been used widely to protect marine abundance, biodiversity, and habitat—to the problem of fisheries management.  Their fundamental assumption: If, as the evidence shows, no-take reserves are good for fish populations, shouldn’t they also be good for fisheries?  Dr. Botsford’s work on the MPAs of coastal California uses spatially explicit mathematical models of fish populations to illustrate that the issue is too complex to allow for such reductions. 

Conventional studies of recruitment in fish populations focus on equilibria dependent upon fraction of lifetime egg production (FLEP).  Put simply, mothers must lay eggs in order to propagate the species.  Thus, the more fishing that occurs, the fewer adult females will be present to lay eggs, the fewer eggs will be laid (representing a lower FLEP), the fewer larva will survive to join the population, and the fewer adults will be alive to breed in the future, further impacting population persistence through positive feedback.  When extensive fishing pushes FLEP below a certain critical replacement threshold (CRT), the population cannot maintain itself and is in danger of collapse.  Conventional fisheries management attempts to use aforementioned changes in fishing to titrate the FLEP of a fishery so as to maximize yield while still providing for population replacement.  Each population is different, though, and it remains unclear in many or most cases exactly what level of FLEP results from management levels of fishing or even what level of FLEP constitutes the CRT for any given population.  Given this uncertainty, it is perhaps unsurprising that we see so many overfished stocks from conventional management.
 
MPAs have been applied with the assumption that they will reduce some of this inherent uncertainty, but Dr. Botsford argues effectively against this conclusion.  Even if MPAs are the active management tool, population sustainability will still be dependent on FLEP—it just becomes a spatially distributed variable.  Within MPAs, FLEP should be equal to 1 (given the presence of appropriate habitat) and populations should increase smoothly; outside reserve boundaries, however, FLEP will be dependent on fishing pressure as it would be in any other waters, subject then to the same uncertainties in determination that affect conventional assessments.  In addition, with such spatial differences in FLEP, dispersal distances of larval and adult fish and geographic distributions of MPAs become vitally important determinants of survivorship, and thus future fecundity and population persistence at large.  Animals with large dispersal distances are more likely to venture beyond the boundaries of an MPA, exposing them to fishing effort and lower FLEP conditions.  Extending MPA coverage by having a larger MPA or, better for high dispersers, an evenly spaced network of smaller MPAs can be expected to lead to higher FLEP and increased population persistence.  But dispersal, however important, is notoriously difficult to measure, and dispersal of larva even more so.  Thus, it appears that the MPA framework does little to resolve the uncertainty surrounding measures of population sustainability any better than do conventional management regimes, and may even add on another layer.  And ultimately, evidence suggests limiting fishing effort may indeed be key: Modeling shows that extremely high levels of fishing outside reserves (producing very low FLEPs in matrix areas) results in population collapse for long dispersers regardless of MPA coverage.

In the end, Botsford maintains, MPAs are no silver bullet for fisheries management.  While they do increase population sustainability and resiliency to the effects of high fishing levels, similar effects can in theory be gained from reducing mortality through conventional management restrictions.  Evidence from modeling also suggests that the increases in fish yields through spillover in areas proximal to MPAs often do not compensate for the reduction in yields engendered by the no-take zone, resulting in a net parity or (more likely) loss of catch.  Finally, as we have seen, MPAs do not resolve the great uncertainty surrounding the measurement or impact of these issues, providing no ready solution to the “Weird Relative’s Will” problem: Even under a system of MPAs, the public is still making blind withdrawals from the bank, leaving itself very open to the possibility of waking up one morning to find it has no assets left to its name.