Paper SC/61/BRG19 Kelp Gulls (Larus dominicanus) feeding on southern right whales (Eubalaena australis) at Peninsula Valdes, Argentina: updated estimates and conservation implications

SIRONI, MARIANO; ROWNTREE, VICTORIA; SNOWDON, CHARLES; VALENZUELA, LUCIANO O; MARON, CARINA

Encuentro; International Whaling Commission Scientific Committeee; 2009

Kelp gulls (Larus dominicanus) feed on the skin and blubber of southern right whales (Eubalaena australis) at Península Valdés, Argentina, notably affecting the whales behavior. We have monitored the frequency of gull attacks from 1995 to 2008, and studied behavioral aspects of these interactions during the 1999 - 2001 right whale calving seasons. Gulls did not direct their attacks evenly among all whale age classes. Mother-calf pairs received 81% of the attacks and were attacked five times more often than juvenile whales. Juveniles spent over half of their time alone, and they received most of the gull attacks while they were solitary. However, the attack rate per hour was highest (5.2) when juveniles interacted with mother-calf pairs and lowest (0.7) when they were with adults. The attack frequency has continued to escalate since it was originally studied in 1984 (Thomas 1988) and 1995 (Rowntree et al. 1998). In 1995, 12% of 5-min intervals collected during focal follows of mother-calf pairs contained attacks at two study sites, compared to 26% in Golfo San José and 25% in Golfo Nuevo in 2008. The proportion of whales with gull marks between 1974 and 2008 increased steadily from 1% of whales in 1974 to 37.8% in 1990, 67.6% in 2000, and 76.8% in 2008. Adult whales have learned to reduce the likelihood of being attacked by gulls by changing their resting posture at the surface, arching their backs to keep them underwater. The local gull population has grown since the 1970 s possibly as a consequence of the refuse available from fishing boats operating at sea and at fishery and urban landfills. In 2005, 2007 and 2008 unusually high right whale calf mortalities were recorded at Península Valdés. It has been suggested that gull inflicted wounds could reduce calf survivorship. We provide recommendations for the management of the gull population to reduce the negative effects of gull harrassment on right whale behavior and to reduce the potential impact of gull attacks on calf health. We present a brief account of the three workshops held to analyze this conservation and management problem in Argentina.