Oceanographer here! Regarding the impact of the Second World War on marine life, it’s important to note that the general reduction in fishing and whaling activity worldwide as a result of global conflict actually provided a **beneficial** effect for targeted fish and whales that outweighed the relatively minor detrimental impacts resulting from some spilled oil here or a few depth charge blasts there.

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With regards to migration, the historical evidence for war-induced changes in the behavior of marine life is unfortunately limited. It is only quite recently that we have become able to closely observe patterns of whale behavior and movement through techniques such as GPS tagging, and older techniques such as aerial observation or citizen science (i.e. reportings of sightings by civilians or mariners) were not as widespread during the 1940s. Often, marine biologists studying fish or whale populations during this time period are actually trying to reconstruct data indirectly, based on sources like fishery catches or newspaper reports of whale sightings.

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It makes sense that changes in the patterns of marine traffic during wartime, not to mention the impact of local fighting itself, would impact the behavior of marine life. However, the direct evidence for this is sparse and could be confounded by other factors. For instance, research showing declines in whale sightings off Hawaii reported in newspaper accounts during the war [1] might be explained by whales’ avoidance of coastal areas in response to heightened naval activity. Alternatively, such a pattern could result from changes in the activities of Hawaiians during the war (as a result of military restrictions and societal shifts), or from changes in newspaper reporting (censorship, focus on war news, etc…)

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As I mentioned at the outset though, marine life likely may have benefited overall from the war. Many records of fishery catches document dramatic declines in fish landings during the war years, particularly in more contested waters such as near the British Isles [2]. Nevertheless, some lower level of fishing activity often continued even in wartime, with the exception of the very most hotly-contested regions such as the English Channel where fisheries became largely closed [3]. Whaling activity also generally declined during the war period [4]. However, reductions in fishing pressures were not uniform, with some regions experiencing increased fishing activity in wartime [3].

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Unfortunately, the relief from human fishing pressures during the war would be fleeting, with fishing activity rapidly intensifying in the postwar era. In fact, overfishing generally increased in the decade following World War II, aided in part by technological shifts partly accelerated by the war such as the increased adoption of diesel propulsion for trawlers [2], advances in navigation and positioning, echo-sounding technology to locate fish, and progress in materials such as plastics, synthetic rubber, and twine [3]. Such technological advances likely also aided the postwar expansion of whaling activity to previously less-exploited waters, such as the Southern Ocean near Antarctica [4].

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So when it comes to the behavior of marine animals, given the data that marine biologists have to work with, it’s difficult to make confident statements about how whales, etc… responded to wartime activity. It’s safe to say that at population scales, whales and fish were generally better-off during the war as a result of reduced fishing and whaling activity. Over the long-term, however, the war likely more than made up for this temporary relief, with postwar fishing activity surging once again, aided by technological innovations that the war itself helped produce and promulgate.

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Postscript: Holm (2012) is a very comprehensive read on WWII’s impact on the global fishing industry and I recommend checking it out to learn more.

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[1] Herman, Louis. (1979). Humpback Whales in Hawaiian Waters: A Study in Historical Ecology. Pacific Science. 33.

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[2] Thurstan, R., Brockington, S. & Roberts, C. The effects of 118 years of industrial fishing on UK bottom trawl fisheries. Nat Commun 1, 15 (2010). https://doi.org/10.1038/ncomms1013

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[3] Holm, Poul. (2012). World War II and the “Great Acceleration” of North Atlantic Fisheries. Global Environment. 10. 10.3197/ge.2012.051005.

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[4] Clapham, Phil & Mikhalev, Yuri & Franklin, Wally & Paton, David & Baker, C. & Ivashchenko, Yulia & Brownell, Robert. (2009). Catches of Humpback Whales, Megaptera novaeangliae, by the Soviet Union and Other Nations in the Southern Ocean, 1947–1973. Marine Fisheries Review. 71.

The problem is that is is somewhat hard to be sure. In some ways, the war, ironically, actually had a positive impact on marine life. The conflict and threat of U-Boat attack saw commercial sea fishing significantly curtailed, which gave fish populations several years to recover, a reprieve that would have been quite impossible otherwise. The impact of fishing is far beyond anything that could come from the impact of war, such as the battles, or sunken vessels and oil spills.

When it comes to whales though specifically, there is a particular complication in that because of the fact there was a war going on, study of them, and their migration patterns, was next to impossible, but it is certainly possible that naval activity would influence their behavior. Looking at the populations around Hawaii, although the evidence is anecdotal, there are at least hints of it, but it is hard to saw what the drop in sightings might be attributed to. Following the decline of the whaling industry in the region, whale populations began to increase in the early 20th century and sightings were a common topic in newspapers. Surveying these reports, Louis Herman found a marked decline during the period of the war, but the root cause is hard to peg. As Herman notes:

[The decline] reflecting the restrictions on activities of Hawaiian residents during World War II and the constraints placed on newspaper reports of any offshore events. However, it may also reflect a real absence of the whales during this period, possibly produced by the greatly increased sea activities in Hawaii during the war.

There are extra datapoints to track though, as the sightings increase again by the late '40s, but then begin a decline again in the late '50s, concurrent with rising population on Oahu and growing urbanization, suggesting the impact of human factors, including increased sea and air traffic and the disturbances it brings with it. More modern studies also bear out that whale populations are sensitive to the ambient noise levels from human activities, and rising human activity can shift their behavior.

But again, as with fish populations, whale populations benefited. Commercial whaling was essentially non-existent during the war. The combined catches of whales in the 1938-39 season was in the range of 50,000, but decreased below 10,000 per season from '39 to '44, so even if, at points, they may have been being bothered by the goings on of humankind during the phase, at the same time, they were being hunted an order of magnitude less than before. I'm not a whale, but it does seem like a pretty nice trade off.

Sources

Closmann, Charles E.. War and the Environment: Military Destruction in the Modern Age. Texas A&M University Press, 2009.

Herman, Louis M. "Humpback whales in Hawaiian waters: a study in historical ecology." Pacific Science. Vol 33, No. 1 (1979).

Schneider, V., & Pearce, D. (2004). "What saved the whales? An economic analysis of 20th century whaling." Biodiversity and Conservation, 13(3), 543–562.