Testing a non‐lethal method for determining the sex of C alifornia halibut, P aralichthys californicus , in non‐spawning condition

The California halibut, Paralichthys californicus (Ayres), is the target of an avid recreational fishery and a nearly US $2 million commercial fishery. It is one of the State of California’s highest priorities for developing a fishery management plan (California Department of Fish and Game 2001). A 2011 stock assessment conservatively found the southern population (from Point Conception, California to the USA-Mexico border) to be depleted to about 14% of its unfished biomass (Maunder et al. 2011). However, this stock assessment lacked the sexspecific data required to be reliable (MacCall et al. 2011). Sex-specific data are necessary because of the vastly different life-history parameters for female and male P. californicus. Most notably, females grow faster, are caught more frequently and mature significantly later than males (Maunder et al. 2011). The most robust study on the subject found mature females as small as 36 cm, 50% of individuals mature at 47 cm and 100% mature at 59 cm. In contrast, the same study found mature males as small as 19 cm, 50% of individuals mature at 23 cm and 100% mature at 32 cm (Love & Brooks 1990). The state’s managing authority, the California Department of Fish and Wildlife (CDFW), plans to gather more data over the next several years to address data gaps and deficiencies identified by the stock assessment peer-review panel before pursuing a fishery management plan. The method most widely used by CDFW to determine sex in P. californicus involves dissection and physical inspection of the gonad due to the lack of external, sexually dimorphic characteristics. However, dissection cannot be used to gather sex-specific landings data from the live commercial fishery or in tag–recapture studies. Other non-lethal techniques for determining sex that have been proven effective in similar species include observing the natural or forced discharge of gametes (St-Pierre 1984); cannulation (Nielsen et al. 2014); sonography (Loher & Stephens 2011); and genetic analysis (Galindo et al. 2011). The first two methods are limited to mature individuals during spawning activity, and the last method requires careful specimen collection and laboratory expenses. In contrast, sonography can be performed quickly on live fish, year-round, regardless of the fish’s sexual development or spawning condition (Shields et al. 1993; Loher & Stephens 2011). Here, the possibility of using veterinary ultrasound (i.e. sonography) to determine sex in P. californicus was tested following Loher and Stephens (2011). To limit the impact of the study on the wild P. californicus population, whole-fish samples were obtained from existing samples, recreational fishers and aquaculture facilities. The CDFW donated 34 frozen P. californicus that had been obtained as by-catch from purse seine vessels targeting sardines. These fish were used to optimise the