Entanglement: an emerging threat to humpback whales in Scottish waters

Conor Rya, Russell Leaper, Peter G.H. Evans, Katie Dyke, Kevin P. Robinson, Gary N. Haskins, Susannah Calderan, Catharina Francisca van Geel, Olivia Harries, Kerry Froud, Andrew Brownlow, Alistair Jack

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Abstract

Entanglement in static fishing gear, especially shellfish creels (pots), is a known source of mortality and injury for humpback whales (Megaptera novaeangliae) with an apparent rise in recent entanglements of this species in Scottish coastal waters. All available sighting records in Scottish waters from 1992 to 2016 were collated to determine the distribution of the species. A subset of sightings with associated boat-based search effort from the west of Scotland indicated the relative abundance of humpback whales in this region was very low (just four sightings from 86,000 km of search effort). Of the 213 incidental sighting records from 1992 to 2016, 5.6 % (n = 12) comprised known entanglements. For the five most recent years (2012 to 2016), this proportion was higher 7.5 % (n = 10). Over half of the known entanglements (n = 7) involved creels, three others were of ropes consistent with creels, and one involved an aquaculture (salmon) pen. Rescue responses to six of the 12 entangled whales resulted in successful disentanglements, although their long-term survival remains unknown. Three of the 12 entanglement cases (i.e. 25%) were fatal. A gamma distribution was fitted to the frequency of humpback whale ‘visits’ based on the number of different days on which humpbacks were reported. From this, the number of unreported visits in inshore Scottish waters was estimated. Based on the minimum number of reported entanglements, the daily probability that a whale that is present in the area would become entangled was estimated at 0.0017. An independent estimate of entanglement risk, using a subset of effort-related sightings and an assumed effective strip half-width, also suggested the same daily entanglement probability. If a whale were to be resident year-round, these estimates would equate to an annual entanglement probability of 0.46. Based on this probability and the observed proportion of fatal entanglements would suggest a fatal entanglement probability of 0.12. This source of mortality alone is an order of magnitude higher than sustainable levels. A positive correlation between the annual estimates of the number of visits and reported entanglements (r=0.79, df=22, p<0.001) suggests that the method for estimating humpback whale days is a valid approach for assessing risk. In the present scenario, Scottish inshore waters could not support a population of humpback whales and these waters currently act as a high mortality sink for the species in the NE Atlantic. The entanglement issue is also a concern for other species, particularly minke whales. Measures to reduce entanglement risk could also benefit the creel fishing industry by minimising the loss of gear.
Original languageEnglish
PublisherInternational Whaling Commission
Number of pages12
Publication statusPublished - May 2016

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    Rya, C., Leaper, R., Evans, P. G. H., Dyke, K., Robinson, K. P., Haskins, G. N., Calderan, S., van Geel, C. F., Harries, O., Froud, K., Brownlow, A., & Jack, A. (2016). Entanglement: an emerging threat to humpback whales in Scottish waters. International Whaling Commission.