What we see is not what there is: estimating North Atlantic right whale Eubalaena glacialis local abundance

Abstract

Aerial surveys can be used to estimate animal abundance, but animals unavailable for detection for portions of the survey can cause biased abundance estimates. Moreover, these biases may be variable owing to changes in behavior. We conducted focal follows to obtain surface and dive times of North Atlantic right whales Eubalaena glacialis in Cape Cod Bay (CCB) and measured the aircraft field of view; these metrics were combined to estimate availability and correct monthly abundance estimates from 1998 to 2017 generated via distance sampling methodology. We used a general least squares model to test for trends in abundance. Availability varied with month (0.27−0.85), likely linked to changes in the depth of copepod food resources. Detection probability varied across the years (0.43−0.87). Sightings per unit effort and counts of whales were significant, but downward-biased indicators of abundance and availability caused changes in bias over the season. Estimated abundance in CCB increased during the study period (4.9 whales yr −1 ), and estimated abundance in peak months increased at a faster rate (10% yr −1 for 1998−2017) than for the overall population (2.8% yr −1 for 1990−2010). Accurate abundance estimates are necessary to monitor long-term changes in abundance of right whales in CCB, to understand the importance of CCB relative to other areas, and improve management strategies to protect this endangered species from entanglements in fishing gear and ship-strikes. Failing to correct for seasonal variation in availability results in substantial and variable underestimation of abundance.

Rapid Climate-Driven Circulation Changes Threaten Conservation of Endangered North Atlantic Right Whales

“…The most notable shift for E. glacialis was a sharp decline in sightings per unit effort (SPUE) in the Bay of Fundy critical habitat (Figure 2a,b). In the western Gulf of Maine habitats, there was a decline in the Great South Channel and an increase in Cape Cod Bay, consistent with recent analyses (Ganley et al, 2019). Increases of C. finmarchicus were largely restricted to the western Gulf of Maine and occurred in spring, while the late summer through winter decreases in C. finmarchicus occurred throughout the Gulf, corresponding to the period of dormancy at depth (Figures 1 and S1).…”

Abundance estimation for line transect sampling: A comparison of distance sampling and spatial capture-recapture models

“…DS has been used to produce spatially explicit estimates of North Atlantic right whale (NARW) abundance [ 26 , 27 ]; however, SCR should be the preferred approach to estimate NARW abundance based on common NARW sampling procedures and SCR’s outperforming DS in our comparisons. NARWs are individually identifiable, and their population is surveyed by aerial line transect surveys conducted over multiple occasions in the same region [ 27 , 29 ]. Additionally, g(0) ≠ 1, because NARWs, like other marine mammals, spend time below the water’s surface where they are unavailable for detection and may be difficult to detect at the surface, depending on environmental conditions and behavior [ 37 , 46 ].…”

“…DS has been used to produce spatially explicit estimates of North Atlantic right whale (NARW) abundance [26,27]; however, SCR should be the preferred approach to estimate NARW abundance based on common NARW sampling procedures and SCR's outperforming DS in our comparisons. NARWs are individually identifiable, and their population is surveyed by aerial line transect surveys conducted over multiple occasions in the same region [27,29].…”

“…Ten horizontal line transects were surveyed each occasion, although each line need not be surveyed every occasion in practice (Fig 1). Line transects were defined as y � (11,13,15,17,19,21,23,25,27,29), with endpoints at x = 10 and 30. The probability that individual i was detected on occasion j, denoted as y i,j = 1, was a function of g(0) and the distance between the individual and the closest transect on occasion j, d detection,i,j , with a scale parameter, σ d , of 1/3.…”

“…One example of a species whose abundance is often estimated with DS but could be estimated with SCR is the endangered North Atlantic right whale (NARW) Eubalaena glacialis [ 26 , 27 ]. Aerial line transect surveys are conducted off the southeastern US coast (SEUS) on most days with suitable weather each winter (December through March) to monitor the NARW population while on their winter calving grounds [ 28 , 29 ].…”