By-Catch of Grey Seals (Halichoerus grypus) in Baltic Fisheries—A Bayesian Analysis of Interview Survey

Abstract

Baltic seals are recovering after a population decline. The increasing seal stocks cause notable damage to fisheries in the Baltic Sea, with an unknown number of seals drowning in fishing gear every year. Thus, sustainable seal management requires updated knowledge of the by-catch of seals—the number of specimens that die in fishing gear. We analyse the by-catch of grey seals (Halichoerus grypus) in Finland, Sweden, and Estonia in 2012. We collect data with interviews (35 in Finland, 54 in Sweden, and 72 in Estonia) and analyse them with a hierarchical Bayesian model. The model accounts for variability in seal abundance, seal mortality and fishing effort in different sub-areas of the Baltic Sea and allows us to predict the by-catch in areas where interview data was not available. We provide a detailed description of the survey design and interview methods, and discuss different factors affecting fishermen's motivation to report by-catch and how this may affect the results. Our analysis shows that the total yearly by-catch by trap and gill nets in Finland, Sweden and Estonia is, with 90% probability, more than 1240 but less than 2860; and the posterior median and mean of the total by-catch are 1550 and 1880 seals, respectively. Trap nets make about 88% of the total by-catch. However, results also indicate that in one of the sub-areas of this study, fishermen may have underreported their by-catch. Taking the possible underreporting into account the posterior mean of the total by-catch is between 2180 and 2380. The by-catch in our study area is likely to represent at least 90% of the total yearly grey seal by-catch in the Baltic Sea.

A Novel Tool to Mitigate By-Catch Mortality of Baltic Seals in Coastal Fyke Net Fishery

“…Researchers took part to the field work (tagging live animals and taking samples from dead ones) during years 2011–2013 and they often had possibility to verify the reliability of the data. Although the fishermen’s motivation to report by-catch has generally been low in Finland [ 18 ], the volume of by-catch reported in this study is high and comparable between the two periods (2008–2010 and 2011–2013). It is therefore likely that the volumes of by-catch detected in our study are not underrated.…”

“…In addition to hunting, by-catch mortality may be another significant component of the anthropogenic mortality of seal populations. Estimated annual by-catch of the Baltic grey seal is in the order of 2000 animals or more [ 18 ] but for ringed seals the magnitude is unknown. An information gap regarding by-catch and its mitigation methods in coastal and small scale fisheries has been recognized globally [ 19 ].…”

“…Considerable efforts have been put into research on interaction between Baltic grey seal and fisheries [ 10 , 14 , 17 , 18 , 21 , 22 ] and on the development of seal-proof fishing gear. For example, trap nets with modified fish chambers, i.e.…”

The Impact of Food Resources, Reproductive Rate and Hunting Pressure on the Baltic Grey Seal Population in the Finnish Sea Area

“…The current knowledge on the level of by-catches of Baltic seal species is limited to a few dedicated studies which suggest that this factor can be substantial. An analysis of reported by-caught grey seals showed that approximately 2,000 grey seals are caught annually in the Baltic fisheries (Vanhatalo et al 2014), but numbers of by-caught ringed seals and harbour seals are not known.…”

A Nietzschean Take on a Hundred-Dollar Bill: Reading Weber’s “Protestant Ethic” in Connection with a Contemporary Economist’s Comments

“…Прилов -неумышленный вылов морских млекопитающих или определённых видов рыб [Soykan et al 2008] -получил относительно небольшое внимание со стороны биологов и экологов [Read, Drinke, Northridge 2006;Vanhatalo et al 2014;Johnston et al 2015]. Однако проведённые исследования показывают, что прилов оказывает значительное влияние на локальные морские экосистемы, сокращая популяции высших хищников.…”