A Bayesian approach for understanding the role of ship speed in whale–ship encounters

Abstract

Mandatory or voluntary reductions in ship speed are a common management strategy for reducing deleterious encounters between large ships and large whales. This has produced strong resistance from shipping and marine transportation entities, in part because very few studies have empirically demonstrated whether or to what degree ship speed influences ship-whale encounters. Here we present the results of four years of humpback whale sightings made by observers aboard cruise ships in Alaska, representing 380 cruises and 891 ship-whale encounters. Encounters occurred at distances from 21 m to 1000 m (x = 567 m) with 61 encounters (7%) occurring between 200 m and 100 m, and 19 encounters (2%) within 100 m. Encounters were spatially aggregated and highly variable across all ship speeds. Nevertheless a Bayesian change-point model found that the relationship between whale distance and ship speed changed at 11.8 knots (6.1 m/s) with whales encountering ships, on average, 114 m closer when ship speeds were above 11.8 knots. Binning encounter distances by 1-knot speed increments revealed a clear decrease in encounter distance with increasing ship speed over the range of 7-17 knots (3.6-8.7 m/s). Our results are the first to demonstrate that speed influences the encounter distance between large ships and large whales. Assuming that the closer ships come to whales the more likely they are to be struck, our results suggest that reduced ship speed may be an effective management action in reducing the probability of a collision.

Factors affecting whale detection from large ships in Alaska with implications for whale avoidance

“…Ship speed has been demonstrated to reduce the chance that a collision is lethal should one occur (Vanderlaan & Taggart 2007) and is thus a key conservation strategy in many areas (Gende et al 2011, Conn & Silber 2013). We did not include ship speed as a covariate in our models because in GBNP speed is spatially autocorrelated with the narrowest areas of the park.…”

“…While these methods are effective in reducing the relative and absolute risk of collisions (van der Hoop et al 2012), they may not always be feasible, such as when the geography of an area is too narrow to provide alternatives to shipping lanes (e.g. Webb & Gende 2015) or in areas where ships approach a port of call and cannot be re-routed around high-use whale habitat (Gende et al 2011, Guzman et al 2012. Even when shifts in shipping lanes are used, they may result in a reduction, but not elimination, of the risk of ship−whale encounters as whale aggregations may shift within and among years with shifts in prey or oceanographic conditions (e.g.…”

“…The current peak seasonal quota is 153 ship entries, and thus most days the daily maximum quota (2 ships per day) is met, although on a number of days either 1 or 0 ships enter the park. The shoulder season quota is 122 ship entries, although this quota is never met as Due to the large number of whales and narrow geography of the park, cruise ship routes overlap high-use whale habitat, resulting in a large number of ship−whale encounters (Gende et al 2011, Harris et al 2012. Lethal cruise ship−humpback whale collisions have been recorded both in the park and in nearby areas (Neilson et al 2012).…”

“…From 2008 to 2015, observers boarded cruise ships during 643 entries to record the frequency and proximity of surfacing events (encounters) of whales near the ships (see Gende et al 2011, Harris et al 2012; Fig. 1).…”

“…Regardless of embarkation location, effort continued until the ship approached either Tarr or Johns Hopkins Inlet, where whale−ship encounters are rare ( Fig. 1; Gende et al 2011), and was reinitiated once the ship began its course back toward the park entrance. The observer continued until either transfer for disembarkation via NPS vessel at the southern end of the park, or until dusk, which generally occurred in Icy or Chatham Strait to the east of Glacier Bay, or into Cross Sound to the west (Fig.…”