Relationship between horizontal hydroacoustic stock estimates and gillnet catches of surface-oriented fish in shallow Lake Balaton (Hungary)

Abstract

Key-words:gillnet, side-looking hydroacoustics, comparability of data, Alburnus alburnus, shallow lakeWe examined whether gillnetting and hydroacoustics provide comparable relative fish density and fish size distribution estimates in the uppermost water layer of a shallow turbid lake and whether the inclusion of environmental and stock parameters could improve comparability of data between different gears. According to gillnetting, most fish shorter than 14 cm in total length were Alburnus alburnus, and most fish longer than 14 cm were Pelecus cultratus. Size distributions and median sizes obtained from gillnetting and acoustics differed. Pure acoustics-derived density estimates accounted for only <8.8% of the variation in the gillnet catch by number and no correlation was found in biomass indices. Most variances observed in the gillnet catch-per-unit-effort data were associated with water transparency, none of the other investigated variables had significant explanatory power. We conclude that the feasibility of establishing a powerful model describing the relationship between fish density estimates of gillnetting and acoustics is low in shallow and turbid habitats, and it might be successful only if the crucial sampling and environmental parameters affecting catch efficiency of gillnets and post-processing of acoustic data are considered. This study cautions again that gillnetting should be used carefully to analyse trends of fish abundance. RÉSUMÉRelation entre les estimations horizontales hydroacoustiques du stock et les captures au filet maillant de poissons de surface en eaux peu profondes du lac Balaton (Hongrie) Mots-clés :filet maillant, hydroacoustique latérale, comparabilité des données, Alburnus alburnus, lac peu profond Nous avons examiné si les filets maillants et l'hydroacoustique fournissent des données comparables de la densité des poissons et des estimations de distribution en taille des poissons dans la couche d'eau supérieure d'un lac peu profond trouble et si l'inclusion de paramètres environnementaux et du stock de poissons pourrait améliorer la comparabilité des données entre les différents engins. Selon la pêche au filet maillant, la plupart des poissons de longueur totale inférieure à 14 cm étaient Alburnus alburnus, et la plupart des poissons de plus de 14 cm étaient Pelecus cultratus. Les distributions de taille et les tailles médianes obtenues à partir des filets maillants et de l'acoustique sont différentes. Les estimations dérivées de la seule densité acoustique ne représentaient que moins de 8,8 % de la variation des captures au filet maillant en nombre et aucune corrélation

Within‐lake distribution patterns of fish assemblages: the relative roles of spatial, temporal and random environmental factors in assessing fish assemblages using gillnets in a large and shallow temperate lake

“…Characteristic vertical trends were found in the CPUE of the two dominant species, total CPUE, mean fish mass, species richness and diversity and per cent composition of samples. The benthic water layer of Lake Balaton could be characterized as an A. brama dominated species‐rich fish assemblage, while the surface water layer was a species‐poor assemblage dominated by A. alburnus (Specziár et al ., ; György et al ., ).…”

“…Species name abbreviations are given in Table I. Table I. water layer of Lake Balaton could be characterized as an A. brama dominated species-rich fish assemblage, while the surface water layer was a species-poor assemblage dominated by A. alburnus (Specziár et al ., 2009;György et al ., 2012).…”

“…Time-related gradients can be separated into diel and seasonal cycles and to between-year components. Random components include a series of uncontrollable phenomena, such as small-scale patchiness of fish distribution and changes of weather, some of which may also affect sampling efficiency (Hansson & Rudstam, 1995;Linløkken & Haugen, 2006;György et al ., 2012). Therefore, within-lake organization of fish assemblages is a consequence of complicated processes driven by environmental heterogeneity (both in time and space) and the differences in ecological features of fish species and their size groups (Jackson & Harvey, 1997;Winfield, 2004;Lund et al ., 2010).…”

“…One of the most popular sampling methods in still waters is gillnetting. Gillnetting is a passive fishing method in which catching efficiency varies between fish species and size groups, depending on stock density, environmental circumstances (weather, water temperature and transparency, habitat structure and feeding conditions) and sampling conditions (net construction, time of day and soak time) (Hamley, 1975;Olin et al ., 2004;Linløkken & Haugen, 2006;Pierce et al ., 2010;Prchalová et al ., 2011;György et al ., 2012). Standardized sampling protocols can control a large part of the gear-related variance in the samples but generally not those linked to random variations in sampling conditions.…”

“…Standardized sampling protocols can control a large part of the gear-related variance in the samples but generally not those linked to random variations in sampling conditions. For example, water transparency and temperature are factors that can significantly influence gillnet catches, and may vary across sampling sites and days, especially in shallow, wind-exposed waters (Hansson & Rudstam, 1995;Linløkken & Haugen, 2006;György et al ., 2012). Although how these factors affect fish assemblage or fish assemblage sample variability has been widely investigated, their relative importance in structuring gillnet samples at the lake level is still poorly understood.…”