Although collisions with motorized ships are a recognized source of whale mortality, little has been done to compile information on the frequency of their occurrence or contributing factors. We searched historical records and computerized stranding databases for evidence of ship strikes involving great whales (i. e., baleen whales and the sperm whale). Historical records suggest that ship strikes fatal to whales first occurred late in the 1800s as ships began to reach speeds of 13‐15 kn, remained infrequent until about 1950, and then increased during the 1950s‐1970s as the number and speed of ships increased. Of 11 species known to be hit by ships, fin whales (Balaenoptera physalus) are struck most frequently; right whales (Eubalaena glacialis and E. australis), humpback whales (Megaptera novaeangliae), sperm whales (Physeter catodon), and gray whales (Escbricbtius robustus) are hit commonly. In some areas, one‐third of all fin whale and right whale strandings appear to involve ship strikes. To assess contributing factors, we compiled descriptions of 58 collisions. They indicate that all sizes and types of vessels can hit whales; most lethal or severe injuries are caused by ships 80 m or longer; whales usually are not seen beforehand or are seen too late to be avoided; and most lethal or severe injuries involve ships travelling 14 kn or faster. Ship strikes can significantly affect small populations of whales, such as northern right whales in the western North Atlantic. In areas where special caution is needed to avoid such events, measures to reduce the vessel speed below 14 kn may be beneficial.

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To reduce right whale Eubalaena glacialis deaths caused by ship collisions along the US East Coast, a rule was implemented on 8 December 2008 requiring all vessels ≥65 feet (19.8 m) to travel 10 knots (18.5 km h −1 ) or less in 10 seasonal management areas (SMAs). To evaluate the effectiveness of this rule, we plotted the locations of all right whale and humpback whale Megaptera novaeangliae carcasses attributed to ship-strikes since December 1990 in US waters to determine their proximity to SMAs. In the 18 yr pre-rule period, 13 of 15 (87%) right whales and 12 of 26 (46%) humpback whales killed by ships were found inside later SMA boundaries or within 45 nmi (83 km) of their perimeters during later active dates. In the first 5 yr after the rule became effective, no ship-struck right whales were found inside or within 45 nmi of any active SMA. This was nearly twice as long as the longest pre-rule period without discovery of a ship-struck carcass in those areas during effective time periods. Based on the 18 yr pre-rule period, bootstrap resampling analyses revealed that the probability of finding no ship-struck whales in or near SMAs during the first 5 yr post-rule period would be a statistically significant reduction in such deaths (p = 0.031). The results suggest the rule has been effective at reducing right whale deaths. We suggest enlarging SMAs to include additional parts of the right whale migratory corridor.

Collisions between ships and whales are reported throughout the world's oceans. For some endangered whale populations, ship strikes are a major threat to survival and recovery. Factors known to affect the incidence and severity of collisions include spatial co-occurrence of ships and whales, hydrodynamic forces around ships, and ship speed. Less understood and likely key to understanding differences in interactions between whales and ships is whale behavior in the presence of ships. In commercial shipping lanes off southern California, we simultaneously recorded blue whale behavior and commercial ship movement. A total of 20 ship passages with 9 individual whales were observed at distances ranging from 60 to 3600 m. We documented a dive response (i.e. shallow dive during surface period) of blue whales in the path of oncoming ships in 55% of the ship passages, but found no evidence for lateral avoidance. Descent rate, duration, and maximum depth of the observed response dives were similar to whale behavior immediately after suction-cup tag deployments. These behavioral data were combined with ship hydrodynamic forces to evaluate the maximum ship speed that would allow a whale time to avoid an oncoming ship. Our analysis suggests that the ability of blue whales to avoid ships is limited to relatively slow descents, with no horizontal movements away from a ship. We posit that this constrained response repertoire would limit their ability to adjust their response behavior to different ship speeds. This is likely a factor in making blue whales, and perhaps other large whales, more vulnerable to ship strikes.