Sunday, July 25, 2010

How to manage Shark Populations

Very interesting paper!

Coming from a fisheries biologist, the intrinsic (and correct!) assumption is that Sharks can be fished sustainably provided that they are being adequately managed. For that, one of the factors that must be taken into account is the capability of depleted stocks to rebound - and when it comes to Sharks, it is obviously not good news.
What I found interesting is to discover that different habitats, life histories and adult sizes have resulted in different species-specific reproductive strategies that need to be considered when formulating those management measures.
Highly recommended reading for everybody!


A demographic technique is used to compare the intrinsic rates of population increase of 26 shark species hypothetically exposed to fishing mortality.

These rates (r2M) are used as a measure of the relative ability of different sharks to recover from fishing pressure.
The method incorporates concepts of density dependence from standard population modelling and uses female age at maturity, maximum reproductive age, and average fecundity. A compensatory response to population reduction is assumed in pre-adult survival to the extent possible given the constraints of the life-history parameters.

‘Rebound’ productivity was strongly affected by age at maturity and little affected by maximum age.
Species with lowest values (r2M <> 0.08) were small coastal, early-maturing species. Sharks with mid-range values (r2M = 0.04–0.07) were mostly large (> 250 cm maximum size) pelagic species, relatively fast growing and early maturing.

Possible selection pressures for these three shark groups, management implications, practical applications for the derived parameter r2M, and recommended areas of research are discussed.


Compared with other marine fishes, sharks have relatively low productivities, but nevertheless there is a wide range among species, which have differing abilities to withstand, or to recover from, exploitation.
The range is at the lower end of the scale compared with most exploited teleosts, many of which have high turnover rates and fecundities (e.g. Pacific sardine, Sardinops sagax, r = 0.34, Murphy 1967). The resulting spectrum of r2M values offers a unique view of the relative productivities of the species examined and should prove useful to managers in considering the intrinsic rebound potentials.

Although we found no obvious patterns by taxonomic category, the results do reveal an interesting pattern along the productivity continuum with regard to adult shark size and certain life-history traits.

In general, there was a tendency for smaller-sized species to mature earlier, to be shorter-lived, and to have higher intrinsic rates of increase r2M than larger species, as expected from ecological and evolutionary theory.

Sharks with the highest rebound or r2M capabilities (r2M > 0.08) were smaller, inshore coastal species
that mature early and tend to be comparatively short-lived (smoothhounds, Mustelus spp., bonnethead, Sphyrna tiburo, and sharpnose shark, Rhizoprionodon terraenovae).
These species may be naturally selected for fast turnover in the protected and predominantly benthic environment of inshore coastal areas, especially shallow bays and channels of estuaries and reefs.
These sharks are undoubtedly vulnerable to predation throughout their lives because of their small size, so have countered by evolving short generation times to outproduce their production losses to predators. By maturing early, they benefit from a higher probability that their offspring will survive to reach maturity and begin reproducing.

Those with the lowest recovery capabilities (r2M < 0.04), tended also to be coastal species but were generally medium to large-sized sharks, slow growing and late to mature (Pratt and Casey 1990) (e.g. leopard shark, Triakis semifasciata; dusky shark, bull shark, Carcharhinus obscurus;C. leucas; seven-gill shark, Notorynchus cepedianus; lemon shark, Negaprion brevirostris; sandbar shark, Carcharhinus plumbeus).
These medium to large-sized coastal species appear selected for a long reproductive life. While slow growth and delayed maturity lowers the probability of a juvenile reaching reproductive age, it also makes possible a long life span, large adult size, and larger, more fit offspring (which the large-sized adult females are capable of producing) (Stearns 1992). This long reproductive lifespan helps to bridge lean years, when vagaries in prey availability may compromise preadult survival, for unlike their smaller counterparts, finding and capturing food in a more open and less predictable foraging environment is probably a greater problem for these sharks than predation. Thus, a multi-year reproductive effort may be essential for ensuring that each adult, on average, successfully replaces itself during its lifetime with at least one other mature adult. As mentioned by Stearns (1992), delayed maturity permits further growth and increases in fecundity with size, as well as initial higher fecundity or more fit offspring. In terms of productivity, this can outweigh advantages of early maturation, but only to the point where fitness gained through increased fecundity or juvenile survivorship is balanced by the fitness lost through longer generation time and lower survival to maturity. The r2M rates for these slow-growing, late-maturing species are similar to the rates of population increase estimated for dolphins and small whales (see Reilly and Barlow 1986; Perrin and Reilly 1984).

The sharks within the mid range of r2M-values (0.04–0.07) were mostly large (> 250 cm maximum size) pelagic species, relatively fast growing (Branstetter 1990; Pratt and Casey 1990) and early maturing. These include the blacktip, Carcharhinus limbatus; grey reef, C. amblyrhynchos; silky, C. falciformis; Galapagos, C. galapagensis; mako, Isurus oxyrinchus; white, Carcharodon carcharias; tiger, Galeocerdo cuvier; and blue shark, Prionace glauca.
These sharks from the middle of the r-continuum seem to be selected for size and predatory capabilities. For those living a pelagic existence in a lean though relatively predictable environment, it seems vital to reach a large size quickly to enable the swimming speeds needed to capture swift prey and to cover large distances within vast ranges in search of widely dispersed prey. Larger animals can travel farther between feeding bouts and with greater energetic efficiency than can smaller animals (Schmidt-Nielsen 1972). These fishes appear to invest early in somatic growth, thus delaying sexual maturity but subsequently living longer. Perhaps the most important management application for these rates is for use in examining and ranking various shark species, and for tailoring management strategies that better address areas of vulnerability for the three life-history types described here.

The slow-growing, late-maturing sharks with lowest r2M -values should be least resilient to fishing mortality, and
protecting their reproductive stock should be the priority.
Their delayed maturities and long life spans suggest that all the years during which each female is reproductive are necessary simply to ensure that at least one pup of each sex survives to adulthood. There should be protection at least during the peak reproductive years, possibly by establishing minimum and maximum size limits based on the size at which females first reproduce and the size at which fecundity declines with senescence or beyond which further survival is low. There may also be benefits from fishing only the oldest adults if they exert a constraining pressure upon population growth. Removals from the more numerous immature stock (many of which will die of natural causes during the extended juvenile phase) may well have less impact than removals of the reproductively valuable, adult females. And for some species, both females and young may need special protection if vulnerable in reproductive or nursery habitats (especially in coastal areas that are highly accessible to fishermen or to environmental disturbance).

The more oceanic species with mid-range r2M-values and relatively fast growth rates should be better able to withstand fishing pressure than their late-maturing coastal counterparts.
Some stocks in this group should also be less prone to depletion because of the greater likelihood of continual ‘seeding’ by conspecifics from other areas within their extensive oceanic ranges. But these species are also among the most marketable and vulnerable to the extensive and productive oceanic fisheries, which sustain high exploitation rates. In addition, there are insufficient data with which to adequately assess the extent of this direct and incidental harvest, no regulations or requirements for reporting the shark by-catch in the Pacific oceanic zone, and little monitoring or management of the by-catch in the Exclusive Economic Zones of most countries (Stevens 1996). For this reason, continued catch monitoring and trend analyses are needed to ensure that harvests do not exceed the rebound capabilities of these species. Also, to definitively assess the impact of exploitation upon these large pelagics, much more needs to be learned about the geographic extent of their populations, their habitats, and how different population segments are related.

The small, inshore coastal sharks appear to have the highest rebound potentials to buffer and recover from fishing mortality.
To some extent these species may also be protected from commercial exploitation by their small size and low yield potential, but the proximity of their habitats to centres of human population also makes their entire populations vulnerable. These species are highly accessible to fishermen and relatively easy to capture. If subjected to direct or indirect harvest, not only should they be fished at levels no greater than their r2M values, but special protection should also be imposed for vulnerable stages of their life cycles, particularly the reproductive stages. And even though their rebound rates are the highest among the sharksexamined in this study (8–14%), these rates are still comparatively low for fishes and actually more comparable to the rates of population increase estimated for certain pinnipeds (see Barlow et al. 1995). Furthermore, these sharks are limited in their compensatory flexibility because their already low average age of female maturity (which limits the option to breed earlier), small adult body sizes, short life spans and small brood sizes interact to limit abilities to increase productivity.


These findings reaffirm that when dealing with elasmobranchs, certain paradigms developed for teleostean fisheries management must be put aside.
For instance,
rather than thinking in terms of maximizing yield, the focus should instead be on ways to preserve reproductive capability, allowing for the survival of adults through their most-contributing reproductive years.

Also, because adult stock size and recruitment are so closely linked, we need to abandon the assumption that recruitment overfishing happens relatively late in the history of a fishery following growth overfishing.

Recruitment overfishing refers to that level of population reduction at which the rate of entry of new recruits into the fishery also begins to decline. But with elasmobranchs, this condition, unless the stock is continually replenished by an influx of conspecifics from adjacent areas, will occur almost immediately because of the strong link between stock size and recruitment of progeny.

Furthermore, unlike ‘r-selected’teleosts, there is little chance of the trend being reversed by subsequent large, successful year classes entering the fishery.

Many sharks, especially coastal species, may require certain protections as a basic condition for continued fishing at any level

PS: Having been outed by Patric (thanks) and before anybody starts to hyperventilate, let me clarify where I stand on the topic of banning all Shark fishing:
  • In general terms and on a global scale, I support sustainable fisheries (for all species), meaning that fishing is OK provided that yields remain below the level at which stocks can replenish. For many species of Sharks, stocks are however already severely depleted, meaning that they must be first allowed to recover and need to be strictly managed thereafter.
  • I fully support regional efforts aimed at banning Shark fishing locally as they offer a counterweight to the global onslaught and preserve pockets of biodiversity in view of a possible (although alas not probable) replenishment of depleted stocks elsewhere.
  • Shark finning is a specific fishing technique that is both wasteful and extremely cruel, and it needs to be completely banned.
  • Trophy fishing for Sharks sucks and the record keeping by the IGFA is totally anachronistic and needs to be reformed.

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