Population estimates of humpback and blue whales made through photo-identification from 1993 surveys off California
Calambokidis, J. and G.H. Steiger. 1995. Population estimates of humpback and blue whales made through photo-identification from 1993 surveys off California. Report to Southwest Fisheries Science Center, National Marine Fisheries Service, La Jolla, California. 36pp.
This report summarizes the photographic identification research conducted in 1993 both from the SWFC (PODS) cruises and those conducted during coastal surveys and provides new abundance estimates and information on whale movements. A total of 128 blue whales and 247 humpback whales were individually identified during the coastal and SWFC survey effort in 1993. Abundances were estimated primarily using the Peterson cap-recapture model with samples consisting of identifications in different years or samples based on survey procedure (SWFC systematic surveys vs. Cascadia's coastal effort). The coefficients of variation for these estimates were calculated both using conventional methods as well as new more conservative jackknife procedure that treated subsamples as all the identifications in a region in a given time period.
The best estimates of blue whale abundance were 2,038 (Jackknife CV=0.33, left side) and 1,997 (CV=0.42, right side) based on comparison of all photographs taken on the 1991 and 1993 SWFC cruises and the identifications from the coastal surveys for the same period. These estimates of about 2,000 blue whales are twice that obtained in previous capture-recapture analyses and are similar to the estimates obtained from line-transect estimates for the California coast.
The best estimate of humpback whale abundance was 597 (CV=0.07) based on all samples obtained from surveys in 1992 and 1993. Abundance estimates using similar pairs of years going back to 1988, increased steadily from 498 to 597, or about 5% per year. These abundance estimates were in good agreement with those obtained using other capture-recapture methods (Jolly-Seber) as well as those obtained using line-transect methods.
Additional data were obtained in 1993 on blue whale movements and distribution along the California coast. Three individual blue whales were photographed in both the 1991 and 1993 SWFC surveys, a surprisingly high number. Although evidence of preferences by some blue whales for coastal and others for offshore waters was again found, several blue whales moved from use of offshore to inshore waters both during a season or between seasons.
Most humpback whales identified during the SWFC cruises were seen in coastal waters with only six identifications made in offshore waters outside the typical range of the coastal surveys (more than 50 nmi offshore). All but one of the six identifications made farther offshore were of whales that had been seen at other times in coastal waters. These findings confirm that most humpback whales use coastal waters and the few seen farther offshore are part of the same aggregation using coastal waters. Although a higher proportion of humpback whale calves were seen in 1993 (4.4 to 5.0%) compared to past years, this proportion remains low compared to studies of humpback whale populations in other areas.