High lacustrine gillnet catchability of anadromous Arctic charr

Abstract

Arctic charr is the only freshwater fish in most of the High Arctic, with both resident, anadromous, and landlocked populations present. The anadromous Arctic charr is particularly highly sought-after, and has been an important food source for people living in Arctic regions. A recent decline in harvest in many areas has been related to overexploitation by extensive gillnetting. Very few studies, if any, have estimated the gillnet efficiency for harvesting anadromous Arctic charr. In this study, all anadromous Arctic charr ascending the lake, Linnévatn, in western Spitsbergen, Svalbard, were marked over a period of 5 weeks, in July-August 2008. Recaptures in the lake were obtained by gillnetting with three mesh sizes (39, 45, and 52 mm measured knot-to-knot) during three periods. The total number of anadromous charr in the lake was estimated by both Schnabel and Petersen mark-recapture methods. The catchability increased significantly with fish size. Anadromous charr in the length-class 40.0-56.9 cm captured by the legal mesh size in Svalbard lakes (52 mm) had a mean catchability of 0.0023 per gillnet per 2 h fishing effort, and the catch quota which is set at 40 charr for Linnévatn by the Governor of Svalbard, would have been filled by less than five gillnets, each used for 6 h. Although this short fishing time may give an overestimation of catchability due to the avoidance of gear saturation, the high catchability values still indicate that anadromous Arctic charr populations may be extremely vulnerable to gillnetting. Hence, in addition to catch quotas set for each population, a restriction on the number of gillnets allowed per harvester should also be considered.

An Assessment Model for a Standard Gill Net Incorporating Direct and Indirect Selectivity Applied to Walleye

“…The SAM estimated density and population at length with high uncertainties. To reduce uncertainties, gear selectivity investigators will need to directly estimate the number of fish at length in one or preferably many populations (e.g., Borgstrøm et al 2010). We assumed that an adjusted gill- 120 net CPE for Walleye was directly proportional to density.…”

Changes in the density and growth of brown trout (Salmo trutta) after intensive removal of sympatric Arctic charr (Salvelinus alpinus) in the sub‐Arctic lake Møkkelandsvatn, Norway

“…Intraspecific lacustrine competition can be an important mechanism regulating recruitment of young brown trout from nursery areas into lakes (Borgstrøm ; Borgstrøm et al. ), and the density of young salmonids has been shown to increase when the number of adult fish decrease (Jensen ; Lorenzen & Enberg ; Grossman et al. ).…”

“…This migration of older year classes probably increases the survival of the remaining younger year classes (in the river), through the effects of reduced inter-and intracohort competition. Intraspecific lacustrine competition can be an important mechanism regulating recruitment of young brown trout from nursery areas into lakes (Borgstrøm 1992a;Borgstrøm et al 2010), and the density of young salmonids has been shown to increase when the number of adult fish decrease (Jensen 1977;Lorenzen & Enberg 2002;Grossman et al 2012). Therefore, the low number of 1-3-year-old trout in the catches in Lake Møkkelandsvatn in 1989 (before the removal of charr) was possibly due to low catchability, but most probably because these age groups were not fully recruited from their natal rivers.…”

A method for estimating fish density through the catches of gillnets

“…Finally, gillnets do not damage bottom ecosystems, unlike trawling gear (Althaus et al, 2009; Kenchington et al, 2001; Olsgard et al, 2008). Gillnet catchability coefficients often used to estimate fish abundance (Borgstrøm et al, 2010; Giacomini et al, 2020). Catchability can be estimated directly comparing gillnet catches to a known population size, e.g., hydroacoustics (DuFour et al, 2019; György et al, 2012; Hansson & Rudstam, 1995) or using a statistical approach (Griffiths et al, 2007).…”