Growth, survival, and recruitment of juvenile shortfinned eels (Anguilla australis)in a large New Zealand coastal lake

Abstract

Annual changes in the size composition and relative abundance of juvenile shortfinned eels (Anguilla australis Richardson) in Lake Ellesmere, New Zealand, were studied. Juveniles (<300 mm) were captured using a small beam trawl from 1994 to 1999. Age and growth rates were determined from otoliths and validated from trends in the length frequency of different year classes. Growth in length was linear from age 1 year onwards. Mean annual growth rates declined in recent years to exceptionally low values, averaging 13.2 mm year -1 (range 6.3-22.9 mm year -1 ), and are probably limited by the lack of food. Juvenile eels had high survival rates, exceeding 0.80 year -1 and were probably close to 1.0 year -1 . Recruitment into this shallow, turbid, brackish lake was variable and was correlated with the duration and timing of the lake's opening to the sea. The decline in growth rates and restricted recruitment in recent years could reduce commercial catches of migrating silver eels for the next 10-15 years.

Changes in trophic linkages to shortfin eels (Anguilla australis) since the collapse of submerged macrophytes in Lake Ellesmere, New Zealand

“…Numerically, common bullies (Gobiomorphus cotidianus) are by far the most abundant fish species in the lake, comprising up to 92% of the fish abundance (Glova & Sagar, 2000). Densities of common bullies estimated from trawling average 0.3 bullies m -2 (Graynoth & Jellyman, 2002). Other abundant species include common smelt (Retropinna retropinna), inanga (Galaxias maculatus), black flounder (Rhombosolea retiaria) and shortfin eels (Glova & Sagar, 2000).…”

“…Details of the trawling method are contained in Graynoth & Jellyman (2002); in brief, a small beam trawl (2 m wide) was towed at night behind a powered boat over a 500 m 2 area at approximately 5 knots. A minimum of 5 trawls were collected each evening, and the contents pooled for selection of fish for dietary and stable isotope analyses.…”

“…We compare these data to previously unpublished benthic invertebrate community data from the time when the lake would have contained macrophytes, and to past dietary studies of juvenile and adult shortfin eels to trace changes in the trophic linkages to top predators that have occurred since the loss of macrophytes in the late 1960s. We hypothesize that loss of submerged aquatic plants in past decades has significantly altered benthic communities and trophic pathways to top predators, and these changes may account for recently reported declines in juvenile shortfin eel growth rates (Graynoth & Jellyman, 2002).…”

“…Recently, there has been concern over the declining status of its fishery, a concern that has added to the concern over the decline in water quality that have occurred over recent decades (Jellyman & Todd, 1998). Recent research has highlighted slow growth rates of juvenile shortfin eels (Anguilla australis) and low abundances of some year classes (Graynoth & Jellyman, 2002). Although recruitment failures may be related to both the timing and magnitude of lake openings to the ocean, which are now manually controlled, slow growth rates are more likely related to complex interactions between food sources, and competition with other species (Jellyman & Todd, 1998).…”

Sodium fluoroacetate (1080) residues in longfin eels,Anguilla dieffenbachii,following exposure to contaminated water and food

“…Eels represent an important recreational and commercial fishery for New Zealand (Jellyman 1992;Graynoth & Jellyman 2002). The total allowable commercial catch (TACC) for the South Island is 420 130 kg, whereas the proposed TACC for the North Island is 686 000 kg (R. Necklen, Ministry of Fisheries pers.…”