Wild Harbour

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To our knowledge, the current study is the first attempt to observe the foraging activity of harbour porpoises around a bottom-gillnet as an actual fishery deployment. Harbour porpoises appeared around the bottom-gillnet set-up every day during the observation periods, and the vast majority of instances of presence around the net did not result in bycatch. However, for the period category BB (on the day prior to a set with a bycatch event), the number of presences and the presence probability were greater than for the period category NB (days prior to sets with no bycatch event). The average of presence duration was about 9 min, and, in some extreme cases, the presence duration exceeded 1 h—too long for a porpoise to be just passing along the net. Based on the results, it is suggested that harbour porpoises often stayed around the net, rather than just appearing as they passed by, even when no bycatch occurred. Akamatsu T, Teilmann J, Miller LA, Tougaard J, Dietz R, Wang D, et al. Comparison of echolocation behaviour between coastal and riverine porpoises. Deep-Sea Res II. 2007;54: 290–297. Though we depart, leaving our cave to the fox or the wild cat or the cleanly badger, this place, this land, is ours, and we, tied to it as the wild creatures are, cannot escape from it by going. But we belong to it, we have given ourselves up to it; no other abode can ever break this strange allegiance. (p. 180)

Connor, R. C., Smolker, R. A. & Richards, A. F. Two levels of alliance formation among male bottlenose dolphins (Tursiops sp.). Proc. Natl. Acad. Sci. 89, 987–990 (1992). Teilmann, J., Larsen, F. & Desportes, G. Time allocation and diving behaviour of harbour porpoises ( Phocoena phocoena) in Danish and adjacent waters. J. Cetacean Res. Manag. 9, 201–210 (2007). Macpherson’s focus on destruction, however, arrives at a similar conclusion. Near the end of the novel the couple make plans to leave their cave:

Materials & methods

Suuronen P, Chopin F, Glass C, Løkkeborg S, Matsushita Y, Queirolo D, et al. Low impact and fuel efficient fishing—Looking beyond the horizon. Fish Res. 2012;119–120: 135–146. Verfuß UK, Miller LA, Schnitzler HU. Spatial orientation in echolocating harbour porpoises ( Phocoena phocoena). J Exp Biol. 2005;208: 3385–3394. pmid:16109898 Schaffeld, T. et al. Diel and seasonal patterns in acoustic presence and foraging behaviour of free-ranging harbour porpoises. Mar. Ecol. Prog. Ser. 547, 257–272 (2016).

Wright AJ, Akamatsu T, Mouritsen KN, Sveegaard S, Dietz R, Teilmann J. Silent porpoise: potential sleeping behaviour identified in wild harbour porpoises. Anim Behav. 2017;133: 211–222. Boström MK, Krog C, Kindt-Larsen L, Lunneryd SG, Wahlberg M. Acoustic activity of harbour porpoises ( Phocoena phocoena) around gill nets. Aquat Mamm. 2013;39: 389–396. Perrin WF, Donovan GP, Barlow J. Gillnets and cetaceans. Rep Int Whal Comm Spec Issue 15, Cambridge; 1994. Kastelein, R. A., Steen, N., Gransier, R., Wensveen, P. J. & de Jong, C. A. F. Threshold received sound pressure levels of single 1–2 kHz and 6–7 kHz up-sweeps and down-sweeps causing startle responses in a harbor porpoise ( Phocoena phocoena). J. Acoust. Soc. Am. 131, 2325–2333 (2012). Calculation of a hazard ratio (HR) [ 47] provides a method for objectively evaluating the relative risk of bycatch. HR is often defined as the ratio of the probability of death to survival. When a bycatch occurs with a constant probability (instantaneous hazard rate, m), the risk of bycatch S( t) at total presence duration t is represented as: S( t) = 1−exp(− mt). This equation means that the longer porpoises are present near a net, the more vulnerable to entanglement they become. Although the duration per presence had not a significant difference, the total duration per hour was obviously different because the number of presences was largely different between BB and NB. However, it is difficult to estimate m due to the small number of present samples, though further research may enable the risk assessment of bycatch.

McDonald, B. I., Johnson, M. & Madsen, P. T. Dive heart rate in harbour porpoises is influenced by exercise and expectations. J. Exp. Biol. 221, jeb168740 (2018).

Macaulay, J. D. J. et al. Implications of porpoise echolocation and dive behaviour on passive acoustic monitoring. JASA ( in Press) (2023).Reynolds, J. E. & Rommel, S. A. Biology of Marine Mammals. (Washington D.C., London: Smithsonian Institution Press, 1999).