This study investigates mixed-species associations among marine mammals in the Southern California Bight, focusing specifically on Risso's dolphins (Grampus griseus). Over a six-year period, aerial surveys were conducted to record the frequency and nature of these associations, revealing that Risso's dolphins were the most commonly observed species in mixed interactions. The results indicate a significant decline in the proportion of Risso's dolphins involved in such associations compared to historical records, possibly due to an increase in their relative abundance. The findings underline the importance of monitoring mixed-species associations to understand marine mammal dynamics and ecological interactions.

Exposure to anthropogenic sound can have a range four dimensions and to statistically describe uncerof negative behavioral and physical effects on tainty within volumes of water space where animals marine species and is of increasing ecological and were predicted to occur during exposure periods. regulatory concern. In particular, the response of By properly accounting for positional error in this marine mammals, and notably the family of cryp- study, it is clear that previous studies using single tic deep-diving beaked whales, to military sonar is median RL estimates drastically underestimate the a timely and complex issue.

The relative occurrence of the 16 most common cetacean species in the Southern California Bight (SCB) was compared from the 1950s through 2012 by examining at-sea sighting and stranding data. While systematic survey and population abundance/density data have been available since the 1970s, prior data were collected opportunistically. Comparisons were made through a literature review and using recent results from 15 aerial surveys conducted in the SCB from 2008 through 2012. We attempted to address inconsistency in type of effort across studies by ranking the relative sighting frequency of the most common species based on the most representative study for each period. This comparison revealed changes in the relative occurrence of some species across the approximately 60-y reviewed period. Since the 1950s, common dolphins have remained the most common “species” of cetacean in the SCB. Risso’s dolphin and fin, blue, humpback, and Bryde’s whales appear to have increased in relative occurrence. The relative occurrence of the common bottlenose and northern right whale dolphins; Dall’s porpoise; and gray, killer, minke, Cuvier’s beaked, and sperm whales do not appear to have changed notably since the 1950s. There is possible indication of recent decreased relative occurrence of the Pacific white-sided dolphin. The short-finned pilot whale has decreased since the 1950s and has not been recorded in the SCB since the 1990s, concurrent with the observed relative increase in Risso’s dolphins. Overall, recent aerial surveys indicate that the 16 most commonly seen species in the SCB, in descending order of frequency, were common dolphins (two species), Risso’s dolphin, fin whale, common bottlenose dolphin, gray whale, blue whale, Pacific white-sided dolphin, humpback whale, northern right whale dolphin, common minke whale, Dall’s porpoise, killer whale, Bryde’s whale, Cuvier’s beaked whale (the latter three tied in ranking), and sperm whale. Given that the reviewed historical data from the 1950s and 1960s are virtually the only sources of information available to examine trends over the last 50 to 60 y in this area, we believe this comparative ranking approach provides useful information not available from other sources.