Old photographs show decline in predatory fish stocks

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Old photographs are giving stark evidence of the drastic decline of large predatory fish being caught from the reefs off the Florida Keys, according to research to be published in the journal Conservation Biology.

The study by Scripps Institute of Oceanography researcher Loren McClanachan examined archival photographs (most taken by professional photographer Charles Anderson) deposited in the Monroe County Library in Key West.

These photos were taken of trophy-sized fish caught in charter fishing expeditions, and through these photographs as well as her own photographs taken in 2007, McClenachan was able to quantify changes in the size structure of reef fish communities, recording a decline in the sizes of 13 groups of trophy fish caught in the period from 1956 to 2007.

Mean sizeCalculating the mean size of the prize catches, including sharks, large groupers and other hefty fish in early photographs, McClenachan was able to chart a decline in the mean size of the trophy fish from 91.7 cm to 42.4 cm from 1956 to 2007.

During the same period, the average weight of the fishes declined from 19.9 kg to 2.3 kg.

As examples, the average sizes of sharks decreased from almost two metres to 91 cm and those of groupers decreased by one metre during the study period.

Analysing the taxonomic breakdown of the landings, McClenachan found the decline in the size of trophy fish to be due to changes in the composition of landings rather than a drop in the mean size of individuals within groups (except in the case of sharks).

Large Epinephelus grouper formed 25% of the catches (by individuals) in 1956; this dropped to 12% by 1979.

Decrease in sizeThe decrease in the size of trophy fish revealed in this study confirm results of previous studies, which show that ...major declines have occurred in populations of large fish in Florida Keys ecosystems and that chronic overfishing was occurring by the 1970s.

McClenachan notes, the continued viability of sport fishing based on increasingly small individuals in a degraded reef environment indicates a decoupling of the health of the marine environment from the value of the marine-based tourism industry.

Interestingly, McClenachan notes that the reef fish communities of the Florida Keys were already disturbed at the start of the study period (1950s), as reef sharks had already undergone decline since the 1930s and 1940s, and large groupers have been commercially fished since the 1880s.

Therefore, pristine coral reef ecosystems supported far more large fish than are implied by these historical photographs.

As another interesting point, the price of these charter-fishing trips remained constant for more than 50 years, despite a decrease of 88% in fish weight. When adjusted for inflation, the price has remained constant at 40"48 (2007) US dollars.

For more information, see the paper: McClenachan, L (2009) Documenting loss of large trophy fish from the Florida Keys with historical photographs. Conservation Biology, doi:10.1111/j.1523-1739.2008.01152.x