Tope (Galeorhinus galeus) COSEWIC assessment and status report: chapter 6

Biology

Current knowledge surrounding life history parameters of Pacific elasmobranchs has been summarized in an online life history matrix assembled by the Pacific Shark Research Centre at Moss Landing Marine Laboratories. The matrix includes up-to-date information on taxonomy, geographic range, age and growth, longevity, reproduction, demography, trophic interactions, habitat utilization, genetics, recruitment, mortality, and behaviour of 102 species.  This matrix was drawn upon as an authoritative summary of the current state of tope knowledge in the northeast Pacific.

There has been no research on tope in Canadian waters. Information from U.S. waters is limited to research undertaken following the extensive fisheries during the late 1930s and early 1940s (Ripley 1946). The most recent and comprehensive biological information on tope is from populations around Australia and New Zealand that are targeted by commercial fisheries and to a lesser degree from populations from the northeast Atlantic.  It should be noted that life history characteristics between ocean basins and/or hemispheres may not be comparable.

Life cycle and reproduction

In the northeast Pacific, a single study indicating that few females have unfertilized eggs by May suggests that fertilization occurs primarily in the spring (Ripley 1946). Little is known about the breeding behaviour of tope. The reproductive cycle for tope is reported as one year in the eastern north Pacific (Ripley 1946), two years in Australia, (Olsen 1954) and up to three years off Brazil (Peres and Vooren 1991). Globally the gestation period is thought to be 12 months (Ripley 1946; Last and Stevens 1994). Tope are ovoviviparous, with females carrying between 6 and 52 pups near term depending on the size of the female (Ripley 1946; Compagno 1984; Ebert 2003). Parturition in the northeast Pacific is thought to occur between March and July with pups being an average 35-37 cm long (Ripley 1946).

Tope from Brazil, Australia, and New Zealand exhibit rapid growth during the first three years followed by steady growth until about 10 years of age and then slow continued growth through maturity (Peres and Vooren 1991; Moulton et al. 1992; and Francis and Mulligan 1998). In the northeast Pacific maximum length of females is 195 cm and 175 cm for males (Compagno 1984). Aging and longevity is constrained due to the difficulty in reading vertebral sections. The aging technique used in Brazil involving X-rays (Peres and Vooren 1991) is considered more reliable than the Australian technique using alizarin staining of the whole centrum (Moulton et al. 1992). Ferreira and Vooren (1991) found Brazilian tope to be slow growing and to reach a maximum age of 40 years, whereas Moulton et al. (1992) reported a faster growth rate and a maximum age of 20 years in Australia. Longevity in Australian tope; however, is estimated to be at least 45 years based on a tagged individual at liberty for 35 years (Moulton et al. 1989).

Age of maturity based on a study from New Zealand, found females to mature at about 13-15 years (and males at about 12-17 years) (Francis and Mulligan 1998). In eastern Pacific waters, females are mature at 150 cm total length and males are mature at 135 cm.

Generation time, which is the average age of parents of the current cohort, is estimated as the age at which 50% of the females are mature+ 1/M where M is the instantaneous rate of natural mortality. Instantaneous natural mortality of eastern Pacific tope has been estimated at 0.113 (Smith et al. 1998). Female tope are 50% mature at ~14 years of age (Francis and Mulligan 1998). Generation time is therefore 14+1/0.113=23 years.

Herbivory/predation

Tope are preyed upon by other elasmobranchs including the white shark (Carcharadon carcharias) and the broadnose seven gill shark (Notorynchus cepedianus) and possibly marine mammals (Ebert 2003). In New Zealand, the killer whale (Orcinus orca) has been reported taking tope off of commercial longlines (Visser 2000).

Dispersal/migration

Movement patterns of tope in the northeast Pacific are poorly understood. Inferences about movement are limited to patterns observed in the California commercial fisheries from 1941-1944 and from a small number of tope tagged.  Overall there appear to be both bathymetric and latitudinal movements that vary by both sex and season (Ripley 1946). Off the northern California coast, Ripley (1946) found that 97.5% of the catch was composed of males (N=5724) whereas off the southern coast 97.8% of the catch were female (N=5020). In the middle the ratios were approximately equal. In southern California tope were most available to the fishery in the spring and summer whereas in northern California they were most abundant between October and December. When depth is considered, it was found that 98% (N=5025) of the females caught in southern California were caught in depths less than 37 m with 87% being caught in waters less than 18 m. In northern California, 97% of the males were caught in waters greater than 37 m, with 40% of the catch taken from waters over 90 m. Overall, female tope were present in southern California in shallow waters during spring and male tope were present in northern California in deep waters during fall.

There have been three tagging studies between southern California and Hecate Strait (N=427, 6 returns). One by the California Division of Fish and Game in 1943 (N=80, 1 return), another by volunteer fishers in California in 1949 (N=38, 3 returns), and finally a two year tagging study carried out by Oregon commercial fishers in 1948 (N=18, 0 returns) and 1949 (N=291, 2 returns) (summarized in Herald and Ripley 1951). Two recaptures of female sharks tagged in California were made ~1600 km away in Canadian waters in Hecate Strait and off the west coast of Vancouver Island after ~3 and 26 months at large respectively (Table 1). Four other recaptures were made between 121– 306 km from their tagging site. One male shark tagged off of Cape Scott, Vancouver Island was caught only two days later in Queen Charlotte Sound (~121 km). These limited results suggest that at least some component of the population undergoes extended migrations and that they are capable of travelling long distances over a short period of time.

In other jurisdictions, tagging studies have been far more extensive. In Australia from the 1940s to the 1990s, a total of 9638 sharks were tagged resulting in 1011 returns (Walker 1999). From 301 tag recaptures during the 1990s 65% of displacements were >500km for large females (>104cm TL), and 20 recaptures for the sexes combined had displacements >1000km with a mean distance between release and recapture positions of 415 km (Walker et al. 1997). The longest recorded displacement is 3016 km for a female (156cm TL at release) released in the Great Australia Bight and recaptured near the southeast coast of New Zealand's South Island after 1033 days. Similarly tagging studies in the northeast Atlantic have also demonstrated extensive movements. Sharks tagged off of England and Ireland were recaptured as far away as Iceland (2416 km), the Azores, and the Canary Islands.

Table 1. Summary of tope tag returns in the northeast Pacific
Source: Herald and Ripley (1951)
Date Tagged
(mm/dd/yyyy)
Sex Study Approximate tagging location Recovery date
(mm/dd/yyyy)
Time/
distance at large
Location of recovery
07/18/1943 F California Fish & Game Ventura, California 09/11/1945 26 months/
1600 km North
Nootka Sound, British Columbia
01/20/1949 M California fishers Baja, California 07/05/1949 5.5 months/
160 km North
San Diego, California
05/18/1949 F California fishers Point Mugu, California 08/29/1949 3.3 months/
1760 km North
Hecate Strait, British Columbia
05/23/1949 F California fishers Malibu Point, California 05/27/1949 4 days/150 km South Encinitas, California
05/07/1949 M Oregon fishers Point Sur, California 08/28/1949 2.7 months/
144 km North
Halfmoon Bay, California
08/05/1949 M Oregon fishers Cape Scott, British Columbia 08/07/1949 2 days/
120 km East
Queen Charlotte Sound, British Columbia

Generalized movement patterns from both Australia and the northeast Atlantic suggest a seasonal bathymetric migration with the sharks moving into deeper water during the coldest months and returning inshore in spring to give birth (Walker 1999).

Interspecific interactions

Ripley (1946) provides the only documentation of tope diet in the northeast Pacific. This shark is an opportunistic predator feeding upon several fish species in both pelagic and demersal environments (Ebert 2003). Items include fish, Clupeidae (Sardinops sagax), Pleuronectiformes-flatfish, plainfin midshipman (Porichthys notatus), Scorpaenidae-rockfishes, Scombridae-mackerel, and Embiotocidae-perches, as well as cephalopods (Teuthoidea) (Ripley, 1946). A recent study in the northeast Atlantic found the diet of adult tope to exist almost entirely of fish (98.8% by weight) (Morato et al. 2003). In Australia teleosts comprised 47% of the diet by weight followed by cephalopods (37%) (Walker 1989). Diet likely varies considerably by season and size of the shark.

Adaptability

The widespread presence of tope throughout the world indicates this species is able to survive in several environments. Tope are likely able to adapt to natural fluctuations in the environment such as changes in prey type and availability. It is unknown how well tope is able to adapt to anthropogenic changes in the environment or drastic changes to population structure due to fishing mortality. 

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