Inputs into stock assessment

Age and growth

Scientists use otoliths (crystalline structures in the inner ear of fish) to find out how old the fish are. When viewed in cross section under a microscope, otoliths show annular rings (like tree rings) that correspond with the fish’s age. [NIWA]

 

Toothfish grow moderately fast. Individuals reach 60 cm TL in about 5 years, 100 cm after 10 years and 150 cm after 25 years. Antarctic toothfish in the Ross Sea have been recorded to a maximum age of 48 years, a maximum length of 2.62 m and a maximum weight of 150.6 kg. However, most fish caught in the region are 10–30 years old and 100–170 cm. Estimates of natural mortality rates are based on the maximum average age in the population – see Table 1, below. [ NIWA]

 

Many toothfish have been aged (> 20,000) to develop annual age-length keys as a way to convert the length distributions observed into age distributions. [NIWA]

The smallest Antarctic toothfish (4–15 cm) have been caught by trawlers fishing for krill and Antarctic silverfish near the Antarctic Peninsula, close to the ocean’s surface while over depths of 3,000–4,000 m.

Juveniles tend to be found on the ocean seafloor in relatively shallow waters (less than 500 m). Both Patagonian and Antarctic toothfish move into deeper waters as they grow older, with the main adult populations occurring in depths of 600–1,800 m. Both species can grow up to 2 meters in length and 100 kg in weight, and feed mainly on fish.

The proportion of fish at length captured in the slope region of the Ross Sea (by sex) shows a stable size structure through time, though the proportions of smaller fish vary depending on where along the slope and the depth that the vessels fish. [NIWA]

Table 1: Summary of biological parameters used in assessments of Ross Sea region Antarctic toothfish.

 

Relationship

Parameter (units)

Male

Female

All

Natural mortality

M (y-1)

0.13

0.13


Von Bertalanffy

t0 (y)

-0.256

0.021



k (y-1)

0.093

0.090



L∞ (cm)

169.07

180.20



c.v.



0.102

Length-weight

(t.cm-1)

1.387e-8

7.154e-9



b

2.965

3.108


Length at maturity (cm)

L50 (±Lto95)

133.2

120.4


Age at maturity (yrs)

A50 (±Ato95)

12.8

16.6


Stock recruitment relationship

h



0.75


σr



0.60

Estimating abundance using mark-recapture

Toothfish do not have a swimbladder, which makes it possible to mark and recapture tagged fish even at the great depths of the Ross Sea. This ongoing tagging programme is unique in the world and has been mandatory since 2004. Toothfish are tagged and released at a rate of 1 fish per ton of fish caught, averaging 1 in 30 fish captured. There is also a tag reward program. [MPI Observer Programme]

Releasing tagged toothfish in the southern Ross Sea

Research subject: AntarcticaFish