Distribution, seasonal abundance and bycatch of ... - Springer Link

Marine Biology (2002) 140: 831–842 DOI 10.1007/s00227-001-0744-y

M.P. Francis Æ C. Duffy

Distribution, seasonal abundance and bycatch of basking sharks (Cetorhinus maximus) in New Zealand, with observations on their winter habitat

Received: 30 March 2001 / Accepted: 30 October 2001 / Published online: 8 January 2002
Springer-Verlag 2002

Abstract Basking sharks occur throughout New Zealand, but are most common in cool temperate latitudes of 39–51S. Inshore records from miscellaneous sources peaked in spring–summer, with few winter records. Two records were of sharks observed in a large brackish water lake. About 203 basking sharks were observed caught by commercial trawlers between 1986 and 1999. Multiple captures were common, including 14 in one tow. Most trawl-caught sharks were taken near or outside the 250 m depth contour, and 91% came from three small regions – East Coast (EC) and West Coast (WC) of South Island and Snares–Auckland Islands (SA). The highest catch (93 sharks) and catch rate (58 sharks per 1,000 tows) were from EC, where sharks were caught only in spring–summer. In SA, sharks were caught mainly in summer, and in WC all were caught in winter. The modal seabed depths for shark tows were 300– 400 m at EC, 700–800 m at WC and 150–250 m at SA. Sharks were therefore caught in the deepest water in winter at WC. It was impossible to determine the actual depths of capture, but circumstantial evidence indicates that most sharks were caught on or near the bottom. The capture of some sharks in midwater in winter argues against hibernation, because hibernating sharks are unlikely to hover in midwater. Males dominated catches in all regions, particularly in WC and SA. In WC and SA, most sharks (94%) were 7–8 m long, whereas in EC most sharks (73%) were 15 m. The greatest recorded headline height was 110 m. For midwater trawl nets, tow location was defined as being on the bottom (if either the start or finish groundline depth was within 1 m of the start or finish seabed depth respectively) or in midwater. Basking shark species identifications were checked, if possible, against photographs taken by observers; one misidentified Somniosus pacificus was deleted. Fifteen tows had reported basking shark catches of £ 150 kg. These were checked against the raw data sheets, and eight were found to have been other species that were incorrectly coded. The remaining seven records were assumed to be misidentifications and deleted. The number of sharks caught in each tow was not always recorded by observers. Multiple sharks were caught in two tows having estimated basking shark catch weights of 45 and 50 t, respectively. We assumed a mean shark weight in these two tows of 5 t, producing an estimated nine and ten sharks, respectively. For all other tows with missing counts, we assumed one shark was caught per tow. This approach is conservative, but probably reasonable, as estimated catch weights for these tows averaged 3.3 t (range 0.6–8 t). Unstandardised catch per unit effort (CPUE) indices were calculated (by dividing the number of sharks caught by the number of tows observed) to identify trends in catch rates by target species, month, depth and year. CPUE values are presented only for strata in which >100 tows were observed. Data for 18 basking sharks caught by research trawlers were extracted from the Ministry of Fisheries Trawl database, and treated in the same way as the scientific observer records. Length conversion We are not aware of any regression equations in the literature for converting fork length (FL) to TL, or vice versa, and we were unable to find any suitable data for developing such regressions. We converted literature reports of FL to TL using TL=1.10ÆFL, determined from the scale drawing in Fig. 1 of Matthews and Parker (1950), which was based on the average measurements of ten sharks.

Results Miscellaneous records We compiled 109 miscellaneous records of New Zealand basking sharks. Of these, 39% were sightings of one or more living animals, 32% were individuals that were caught by fishers (mainly using set net, though a few were shot, harpooned or trawled), 18% were observed during aerial surveys and 11% were strandings. Excluding the sharks recorded during the aerial surveys, 84% of the miscellaneous records for which a date was available (N=75) were from September–February (spring–summer) (Fig. 2). Most miscellaneous records were from the southern North Island and the north-East Coast of South Island (Fig. 1). Only four records were from the northern North Island, and none was from the West Coast of South Island. Regions with the highest frequency of records were New Plymouth, Hawke Bay, Cook Strait, Kaikoura and Banks Peninsula (Fig. 1). Nearly all of these records were within a few kilometres of the coast. The earliest published account was of a 10.43 m shark

833 Fig. 1 Map of the New Zealand region showing the locations of Cetorhinus maximus recorded by scientific observers aboard commercial trawlers, and research trawlers, and from miscellaneous sources. Boxes indicate the main regions where basking sharks were observed aboard commercial trawlers (EC East Coast; WC West Coast; SA Snares–Auckland Islands)

caught in north-eastern New Zealand (3639¢S; 17445¢E) in 1889 (Cheeseman 1891). Cheeseman (1891) reported that several basking sharks were seen in the same region every spring. Several stranded sharks had been caught in fishing gear, as indicated by net or rope markings on the body, presence of attached fishing gear, or removal of the fins or liver. Incidental captures of basking sharks by bottom trawlers were also reported (Fenaughty and O’Sullivan 1978; Paul et al. 1983; Tennyson 1992). Between October and December 1997, two commercial bottom trawlers caught 32 basking sharks on the seabed in depths of 200–300 m off Hawke Bay (C. Robinson, Pacific Trawling, Napier, personal communication). Vessels from the same company fished the same area of flat continental shelf and slope at the same time of year for

10 years before and 1 year after 1997, but did not catch any other sharks. Basking sharks were frequently recorded within 8 km of the coast along the south side of Banks Peninsula and off Lake Ellesmere during aerial surveys of Hector’s dolphins (M. Rutledge, Department of Conservation, Christchurch, personal communication). Most sightings were of single sharks, but in February 1993, groups of >50 and >100 were seen. Some of these sharks were within 100 m of shore, just outside the surf zone. On one of these occasions, groups of two and three sharks were engaged in close nose-to-tail following behaviour as described by Harvey-Clark et al. (1999) and Sims et al. (2000). Tennyson (1992) reported that a coastal freighter steamed for 20 km through a large aggregation of basking sharks south of Banks Peninsula in January 1992.

834

Fig. 2 Cetorhinus maximus. Seasonal distribution of miscellaneous records. Some records represent multiple sharks

Ryan (1974) reported a 5 m basking shark from Lake Ellesmere, a large, shallow, brackish lake south of Banks Peninsula (see Fig. 4), and, in September 1979, multiple sharks were observed in the same lake over a 1 week period, including 21 individuals on a single day (Dodgshun 1980).

Scientific observer records About 203 sharks were reported by observers on commercial trawlers between 1986 and 1999. This number is probably an underestimate, because sharks were not always counted. One-quarter of the tows that caught sharks contained more than one shark (Fig. 3). The greatest number in one tow was 14; this observation was confirmed from the estimated lengths (4.0–6.5 m) and sexes (eight males, six females) recorded by the observer. A catch of seven sharks was supported by individual length (3.5–5.0 m) and weight (1.5–2.0 t) estimates. Three other tows that each caught five sharks were also supported by individual length measurements and sex data. Catches of five or more basking sharks per tow were reported from all three main capture regions (see below), and multiple catches were reported in most months. There was no relationship between the number of sharks caught in a tow and headline height, tow duration, towing speed, time of day, or seabed depth. Most sharks were caught near or outside the 250 m depth contour, and 90.6% came from three small regions – East Coast (EC) and West Coast (WC) of South Island and Snares–Auckland Islands (SA) (Figs. 1, 4, 5, 6; Table 1). No sharks were observed north of 40S, and only one was recorded from near Chatham Islands (Fig. 1). In the remainder of this section, we focus on the three main regions and their main target fisheries.

Fig. 3 Cetorhinus maximus. Frequency distribution of the number of sharks per tow for observed commercial trawl tows that caught at least one shark

East Coast Most sharks (94.6%) were taken by fisheries targeting hoki (Macruronus novaezelandiae) and barracouta (Thyrsites atun) (Table 1; Fig. 4). Overall CPUE was much higher in the barracouta fishery (57.6 sharks per 1,000 tows) than in the hoki fishery (16.2 per 1,000 tows). All crotures were reported between September and March (spring–summer), despite good observer coverage in all months except July and August (Fig. 7). Basking sharks were caught over mean seabed depths of 143–611 m, with a peak at 300–400 m (Fig. 8). Because of the different depth ranges of the target fish species, sharks caught in barracouta tows came from shallower water (143–303 m) than those from hoki tows (281–611 m). It is not possible to determine the depth of capture for any of the sharks, because they may have been caught near the surface while the net travelled to or from the seabed. Most sharks (80%) were caught by bottom trawl tows, and the remainder (20%) by midwater trawl tows on the seabed (Fig. 9). A G-test of the data for these two gear types (midwater trawl tows in midwater were excluded because of the zero observed frequency of sharks) showed a significant but weak association between the number of sharks caught and gear type, with more sharks than expected being taken by midwater trawl tows on the seabed (G1=5.81, P=0.02). An EC target fishery for arrow squid operated in a similar depth range to that for barracouta (mainly 100– 300 m), but was later in the year (96% of observed tows were in April–June). A reasonable number of tows were observed (N=407, compared with 538 in the barracouta fishery), but no sharks were recorded. The annual catches of basking sharks were highly variable. Greatest numbers and highest CPUE were

835 Table 1 Cetorhinus maximus. Numbers of sharks reported by scientific observers and numbers of observed trawl tows, classified by target fish species (EC East Coast; WC West Coast; SA Snares–Auckland Islands) Target species

Hoki, Macruronus novaezelandiae Arrow squid, Nototodarus sloanii Barracouta, Thyrsites atun Silver warehou, Seriolella punctata Spiny dogfish, Squalus acanthias Gemfish, Rexea solandri Jack mackerel, Trachurus spp. White warehou, Seriolella caerulea Ling, Genypterus blacodes Red cod, Pseudophycis bachus Orange roughy, Hoplostethus atlanticus Hake, Merluccius australis Oreos, Oreosomatidae Other species Total

Number of tows observed

Number of basking sharks

EC

WC

SA

EC

WC

SA

3,525 407 538 110 89 3 53 2 0 59 89 0 1,472 41 6,388

20,195 0 474 9 0 54 343 0 2 0 9 432 0 26 21,544

1,794 14,503 478 86 18 14 511 0 54 0 262 41 66 32 17,859

57

39

2 49

recorded in the 1987–1988, 1990–1991 and 1997–1998 July–June years (Fig. 10); July–June years were used because catches in each spring–summer basking shark ‘‘season’’ spanned two calendar years. Only a small percentage of the observed sharks were measured (24%) or sexed (26%). Furthermore, measured sharks were often not sexed, and vice versa. Most sharks (63%) were males, but EC had the highest percentage of females of all three regions (Fig. 11). Measured sharks ranged from 4 to 9 m, but most were