PRETI ET AL.: FEEDING HABITS OF COMMON THRESHER SHARK CalCOFl Rep., Vol. 42, 2001 FEEDING HABITS OF THE COMMON THRESHER SHARK (ALOPIAS VULPINUS) SAMPLED FROM THE CALIFORNIA-BASED DRIFT GILL NET FISHERY, 1998-1 999 ANTONELLA PRETI SUSAN E. SMITH AND DARLENE A. RAMON California Department of Fish and Game National Marine Fisheries Service, NOM 8604 La Jolla Shores Dnve Southwest Fisheries Science Center La Jolla, California 92037 P.O. Box 271 sharksharkshark@hotniail coni La Jolla, California 92038 ABSTRACT (Compagno 1984). It is epipelagic, gregarious, and cos- The diet of common thresher shark (Alopius vulpinus) mopolitan, and in the northeastern Pacific seems to be from US. Pacific Coast waters was investigated by means most abundant within 40 miles of shore (Strasburg 1958). of frequency of occurrence, gravimetric and numerical Its known range extends from Clarion Island, Mexico, methods, and calculating the geometric index of im- north to British Columbia; it is common seasonally from portance (GII) of prey taxa taken from stoniachs col- mid-Baja California, Mexico, to Washington state.' It lected by fishery observers from the California-based is the leading commercial shark taken in California, drift gill net fishery. Sampling was done from 16 August where it is highly valued in the fresh fish trade (Holts et 1998 to 24 January 1999, a time when the California al. 1998). It is also sought by recreational anglers for its Current was undergoing rapid change from El Niiio to fighting ability as well as food value, especially in south- La Niiia conhtions. Of the 165 stomachs examined, 107 ern California. Patterns of observed catches and results contained food representing a total of 20 taxa, revealing of limited tagging suggest that it undertakes a seasonal a broader trophic spectrum than previously reported for north-south migration along the Mexico-U.S. West this species. Of the identifiable items, northern anchovy Coast, moving northward in summer, then returning to (Engraulis mordax) was the most important in the diet waters off Mexico in winter (Hanan et al. 1993). (GI1 = 48.2), followed by Pacific hake (Merluccius pro- Anecdotal accounts identifying prey items of this shark ductus; GI1 = 31.2), Pacific mackerel (Scomber japonicus; are scattered throughout the literature, but no compre- GI1 = 24.8), and Pacific sardine (Savdinops sagax; GI1 = hensive study of food habits has been undertaken. In 9.2). Of the invertebrates, squid (Teuthoidea, including California, as in other parts of the world (e.g., Spain; Loligo opalescenr; GI1 = 6.3), and pelagic red crab Moreno et al. 1989), A. vulpinus frequently occur in as- (Pleuroncodes planipes; GI1 = 6.6), were also important, sociation with large schools of small fishes, and feed on especially numerically. For sharks collected north of 34% them near the surface, often slashing the water with their latitude, hake was the most important identifiable species whiplike tails, presumably to herd or disorient their prey. in the diet; northern anchovy was most important in According to Compagno (1984), this shark also feeds the south, but was not identified in stomachs collected on mackerels, bluefishes, clupeids, needlefishes, lancet- north of Point Conception. fishes, and lanternfishes, as well as squids, octopuses, pelagic crustaceans, and (rarely) seabirds. Although he INTRODUCTION provided no supporting data, Bedford (1992) reported The Pacific Fishery Management Council recently that, unlike other pelagic shark species off California, included the common thresher shark (Alopias vulpinus) the common thresher shark does not appear to be an as a management unit species within the U.S. West Coast opportunistic feeder, but rather feeds almost exclusively Highly Migratory Species Fishery Management Plan, on northern anchovy (Engvaulis mordax). California fish- now under development. This has prompted the need ermen have reported finding salmon in the stomachs of for biological information on life history, stock struc- large individuals (w. Rendernick, Monterey, Calif., pers. ture, feeding ecology, and essential habitat of this species comm. 2/28/98). In the eastern North Atlantic, Pascoe to better assess stock status and harvest impacts. To date, (1986) examined teleost otoliths from the stomach of a little has been documented on its habitat requirements, 264 kg female, and concluded that the stomach had orig- and only anecdotal accounts are available on its feeding inally contained at least 28 scad (Tvdchurus tr~churus),6 ecology off the U.S. West Coast. whiting (Merlangius merlungus), and a single mackerel The common thresher shark is a large, active, and (Scombev scombrus). strong-swimming shark that occurs in neritic and oceanic waters in subtropical and temperate seas worldwide 'Smith, S. E., R. C. Rasmussen, D. A. Ramon, and G. M. Cailliet. Biology and ecology of thresher sharks (fami1y:Alopiidae). In Sharks of the open ocean, E. [Manuscript received 4 February 2001 .] Pikitch and M. Camhi, eds. MS submitted to Blackwell Scientific Publications. 145 PRETI ET AL.: FEEDING HABITS OF COMMON THRESHER SHARK CalCOFl Rep., Vol. 42, 2001 We examined and analyzed the stomach contents of 4. Unmeasurable body parts only: parts cannot be re- common thresher shark collected by drift gill net ob- constructed to obtain standard measurements, but servers off California, and compared diets between higher taxon or species group still identifiable; size/age classes, seasons, and general catch locations. 5. Digested: identifiable only to a very general high- MATERIALS AND METHODS level taxon; and 6. Fully digested: unidentifiable material; slurry. Sampling at Sea Prey itenis were then separated, identified to lowest Stomach samples were collected from three common possible taxonomic level, and enumerated, measured (to thresher shark size groups by federal fishery observers nearest mm, standard length) and weighed (to the near- aboard commercial drift gill net vessels operating off est 0.1 g), when possible. Fish otoliths and squid beaks California and southern Oregon during the 1998-99 were counted in pairs, with the highest count repre- August through January fishing season. Because sampling senting the minimum number present. Weights were time and freezer space are limited aboard these vessels, recorded by taxon groups (not individually), while lengths and to maximize sampling for small fish in the under- of all intact individuals within a taxon were measured. sampled inshore areas, observers were instructed to col- Content data were pooled for all stomachs (all strata lect stoniachs according to the following protocol: combined) and analyzed by prey taxa for relative mea- 1. size group = <100 cni fork length (up to -200 cni sures of prey quantity (RMPQs) as follows: percent nu- total length, or young-of-year), up to 10 stomachs meric occurrence ("/,A?, percent weight (o/oLV'), and per trip;' percent frequency of occurrence ('XF) of food items. The value '%A' = the number of individuals of one prey 2. size group 101-160 cm FL (200-300 cm TL, or ju- taxon divided by the total number of all prey individu- veniles/subadults), up to 5 stomachs per trip; and als x 100; %W = weight of one prey taxon divided by 3. size group >160 cni FL (over about 300 cni TL, or total weight of all prey x 100; and %F = number of adults/large subadults), up to 5 stomachs per trip. stomachs containing prey of one taxon divided by total Samples were excised at sea; esophageal and pyloric number of stomachs that contained any prey items x 100. ends were secured with plastic cinch ties; and the stom- Empty stomachs and certain sinall incidentally ingested achs were bagged, labeled, and frozen. Data on set and organisms, slurry, and detritus were not used in calcu- haul time, water depth, sea-surface water temperature, lating percentages or indices. location, fish size, sex, and maturity state were recorded. Cumulative prey curves were constructed to deter- mine whether an adequate number of speciniens over- Processing in the Laboratory all or in subsamples had been collected to describe diet Stomach samples were thawed, tamped with absorbent (e.g., Hurtubia 1973; Cailliet et al. 1986; Gelsleichter et paper to remove excess water, and weighed full. Contents al. 1999; Yaniaguchi and Taniuchi 2000). The order in were then removed, and the empty stomach was weighed which stoniachs were analyzed was randomized 10 times, to determine the overall weight of the contents. Materials and the mean number of new prey species was cuniu- and slurry were rinsed and sorted with a series of screen lated consecutively in order of the stomachs examined. sieves with mesh sizes 9.5 iiini, 1.4 nini, and 0.5 niin In this type of sample-size analysis, presence of an asymp- for ease in rinsing inidsized food boluses without losing totic relationship indicates that the number of stomachs some of the smallest items, such as fish otoliths. analyzed is sufficient to represent the diet of a particu- Percentage of stomach fullness (0-1 00%) was estimated lar predator, and that enlargement of the sample beyond visually as a broad gauge of relative fullness. The degree the point of curve stabilization would cause no further of prey digestion was estimated as follows: increase in trophic diversity (Hurtubia 1973). 1. Fresh: head, body, shn, and most fins intact, although Measure of prey quantity (RMPQ) values were used some individuals may be in pieces (i.e., bitten on to calculate the geometric index of iniportance (GII), capture) ; as developed by Assis