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Many over‐exploited marine ecosystems worldwide have lost their natural populations of large predatory finfish and have become dominated by crustaceans and other invertebrates. Controversially, some of these simplified ecosystems have gone on to support highly successful invertebrate fisheries capable of generating more economic value than the fisheries they replaced. Such systems have been compared with those created by modern agriculture on land, in that existing ecosystems have been converted into those that maximize the production of target species. Here, we draw on a number of concepts and case‐studies to argue that this is highly risky. In many cases, the loss of large finfish has triggered dramatic ecosystem shifts to states that are both ecologically and economically undesirable, and difficult and expensive to reverse. In addition, we find that those stocks left remaining are unusually prone to collapse from disease, invasion, eutrophication and climate change. We therefore conclude that the transition from multispecies fisheries to simplified invertebrate fisheries is causing a global decline in biodiversity and is threatening global food security, rather than promoting it.

Despite the current interest in using closed areas for fisheries management, few studies have actually examined the benefits for invertebrate fisheries such as scallops. This study details the dynamics of a population of great scallops Pecten maximus (L.), within a closed area and an adjacent fished area off the Isle of Man, over a 14 yr period (1989 to 2003). Scallop densities were very low in both areas when the closed area was set up, but increased at an accelerated rate over time within the closed area. Scallop densities also increased on the adjacent fishing ground, but not to the same extent. Consequently, the density of scallops above the minimum legal landing size (110 mm SL) was more than 7 times higher in the closed area than in the fished area by 2003. There was also a shift towards much older and larger scallops in the closed area and, correspondingly, lower estimates of total mortality. Experimental dredging of 2 plots within the closed area confirmed that fishing drove these differences in population dynamics and structure. These patterns of scallop density, age and size structure resulted in the exploitable biomass (adductor muscle and gonad) of scallops being nearly 11 times higher in the closed area than in the fished area by 2003, and the reproductive biomass was 12.5 times higher. This is significant for fisheries management because the build up of high densities of large P. maximus individuals enhanced local reproductive potential and therefore the likelihood of export of larvae to the surrounding fishing grounds. Along with these direct benefits of closed area protection, juvenile scallops had higher survival and individual growth rates in the closed area, apparently in response to reduced fishing disturbance. Although juvenile scallops are not subject to direct removal by fishing, protection during this critical phase therefore appeared to assist the recovery of the closed area population. In summary, this study joins a growing number indicating that the use of closed areas offers a range of benefits over more traditional methods of managing fisheries. Fisheries for relatively sedentary and long-lived species such as P. maximus appear to be particularly suitable for this type of management.

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Many fish species are cryptic by nature or show a negative response to divers. This may make traditional censusing techniques difficult to perform and the results obtained using these methods may be inaccurate. This problem will be exacerbated in complex habitats, such as those found on coral reefs, because the cryptic species present will be even more difficult to locate and identify. Predation studies on coral reefs have been especially hampered by this problem because many predatory species are cryptic and so it has been difficult to obtain reliable abundance estimates. This study tested a visual census technique that used bait to bring cryptic predatory fish into view. Results from this census were then compared to those from a traditional survey using belt transects, and to a patch reef tagging study where all individuals of 3 key species were located. The baited technique produced significantly higher density estimates for 3 of the 4 most abundant cryptic species. The patch reef experiment demonstrated that the baited technique accounted for 85 to 96% of fish present. Censuses without bait observed only 40 to 61% of fish present. For mobile species, on the other hand, the baited census appeared to overestimate abundance, due to movement of fish into the census area. We therefore recommend combining baited censuses of sedentary cryptic species with traditional censuses of mobile species to gain an accurate picture of piscivorous reef fish communities. Using this approach at Lizard Island on the Great Barrier Reef, the proportion of cryptic piscivores in the community was found to be almost double that which would have otherwise been observed. We also found considerable spatial variation in the abundance and distribution of piscivorous fish. These patterns would have been quite different had the survey been based on belt transects alone. Previous studies using belt transects may therefore have underestimated the importance of cryptic piscivorous species in communities of coral reef fish.