By Andrew J. Read
Source: SeaWeb eNewsletter 
Marine mammal bycatch has declined significantly in U.S. waters over the last decade, but global bycatch of marine mammals remains in the hundreds of thousands, according to a paper in the journal Conservation Biology.
The paper, by Andrew J. Read of the Duke University Marine Laboratory, and colleagues, notes that "fisheries bycatch poses the single greatest threat to many populations of marine mammals in the United States and elsewhere," but that "our knowledge of the global extent, nature, and impacts of direct interactions between marine mammals and fisheries is fragmentary."
The authors "obtained estimates of bycatch for 125 stocks of marine mammals in U.S. waters from published stock assessment reports." They "compiled information on bycatch for each stock from 1990 to 1999 and combined bycatch into three categories of fisheries: gillnets, trawls, and other (e.g., longlines, purse seines, traps" and "stratified these data by the three geographical regions in which they were collected: Atlantic (including the Gulf of Mexico), Pacific (including Hawaii), and Alaska."
Read and colleagues found that the mean average annual bycatch of marine mammals between 1990 and 1999 was 6,215, with bycatch of cetaceans (3,029) and pinnipeds (3,127) occurring in similar numbers. Dolphins and porpoises constituted almost the entire cetacean bycatch, with larger whales accounting for only roughly 20 per year. The majority of cetacean (84%) and pinniped (98%) bycatch was from gillnet fisheries.
There was a significant downward trend in the annual bycatch of cetaceans over the decade, with gillnet totals falling from 4,902 in 1990 to 1,051 in 1999. Conversely, average annual bycatch figures for pinnipeds remained fairly constant: 1,921 in 1990, and 2,344 in 1999. The authors argue that "some of these general trends can be explained by examining the bycatch of particular species. For example, the Gulf of Maine population of harbor porpoises (Phocoena phocoena) was subject to the largest known bycatch of any marine mammal in the United States during the early 1990s. This bycatch declined dramatically from 2,900 in 1990 (57% of cetacean bycatch) to 332 in 1999 (19% of cetacean bycatch). Several mitigation measures, including time-area closures and the use of acoustic alarms, were used in the late 1990s to reduce this bycatch."
Read and colleagues also extrapolated the U.S. figures to estimate global totals, yielding an annual bycatch estimate of 653,365 marine mammals worldwide. While recognizing that this figure is crude, they also suspect that it is an underestimate, primarily because it is based on numbers of fishing vessels worldwide, and the registry of fishing vessels in the [United Nations Food and Agriculture Organization] database is incomplete.