studied in different parts of a subarctic river in northernmost Finland, the River Teno: in ... their new habitat. ...... tion by brook cham in a New Hampshire stream.
Copyright 0 Munksgaard 1998
Ecology of Freshwater Fish 1998: 7: 13-24 Printed in Denmark . All rights reserved
ECOLOGY OF FRESHWATER FISH ISSN 0906-6691
Feeding.of Atlantic salmon, Salmo salar L., parr in the subarctic River Ten0 and three tributaries in northernmost Finland Erkinaro H, Erkinaro J. Feeding of Atlantic salmon, Salmo salar L., parr in the subarctic River Ten0 and three tributaries in northernmost Finland Ecology of Freshwater Fish 1998: 7: 13-24. 0 Munksgaard, 1998 Abstract Available food and feeding of Atlantic salmon parr were studied in different parts of a subarctic river in northernmost Finland, the River Teno: in the large main stem, in a major tributary influenced by lakes, and in two minor tributaries. The highest density of benthic organisms was found in an outlet of a lake, but drifting organisms were the most abundant in the small tributaries. Migrant parr that move to the small tributaries from the main stem have better food resources in their new habitat. Plecoptera nymphs were the most important prey for salmon parr early in the spring. Later in the summer dipterans were usually the dominant prey group. In September the importance of Plecoptera was emphasized in the small tributaries and the lake outlet, and that of Trichoptera in other sites. Drifting prey was used more by large than by small parr. The utilization of drifting food appeared to be related to the water temperature rather than to the availability of the drift.
H. Erkinaro, J. Erkinaro River Tenojoki Fisheries Research Station, Finnish Game and Fisheries Research Institute, Utsjoki, Finland
~
Key words: food availability; drift; benthos; prey selection; habitat selection; salmonidae J. Erkinaro River Tenojoki Fisheries Research Station, Finnish Game and Fisheries Research Institute, FIN-99980, Utsjoki, Finland
Accepted for publication September 15, 1997
Un resumen en espaiiol se incluye detras del texto principal de este articulo.
Introduction
The biomass and growth of juvenile salmonids in running waters are believed to depend chiefly on available food supply when other biotic and abiotic factors allow tolerable conditions in the environment (Waters 1988, Richardson 1993). According to the river continuum concept (Vannote et al. 1980), the production of a riverbed in different reaches of a river system vary predictably, and organic carbon input from allochtonous and in situ production correlate highly with the stream order (Naiman et al. 1987). Habitat selection of stream fish, especially stream-dwelling salmonids, has been subject to considerable research, but most of the studies are primarily confined to relatively small spatial scales within the river systems (Schlosser 1993). At larger spatial scales, the macrohabitats of a system, such as different stretches of the main stem, differently sized tributaries, lacustrine parts of the systems, and estuaries, also have variable habitat quality (Power 1973, Hutch-
ings 1986, Cunjak 1992, Gibson 1993, Erkinaro & Niemela 1995). Moreover, the increasing evidence of substantial movements and habitat shifts of freshwater salmonids, reviewed recently by Gowan et al. (1994), show apparent, active searching for better opportunities for feeding, growth and survival, over relatively long distances between different habitats of watercourses (Jonsson 1985, Naslund 1990, Erkinaro 1995). The first objective of this study is to compare seasonal changes in food availability, both in drift and benthos, and diet of Atlantic salmon, Salmo sular L., parr in different parts of a subarctic river system: a large main stem, a major tributary influenced by lakes, and two minor tributaries where parr enter from the main stem (Erkinaro 1995). The relationships between available food resources and feeding of salmonids in running waters have been widely studied to analyze preferences in position choice (Wankowski & Thorpe 1979, Fausch 1984, Hughes 1992) and to relate feeding to the abundance of invertebrate drift (Elliott 13
Erkinaro & Erkinaro 1973, Bannon & Ringler 1986, Forrester, Chace & McCarthy 1994, Filbert & Hawkins 1995) or to the production of zoobenthos (Gibson et al. 1984, Bergersen 1989). Most of the studies suggest opportunistic feeding behavior for fluvial salmonids (see also Thomas 1962, Thonney & Gibson 1989, Power 1992), although selection of the size and availability of the prey has been noted as well (Allen 1941, Wankowski 1979, Bannon & Ringler 1986, Forrester et al. 1994). On the other hand, changes in environmental conditions, e.g. weather or water temperature, have been noted to affect the type of feeding behavior (Stradmeyer & Thorpe 1987). The second aim of this study is to focus on prey selection and test the opportunism of salmon parr, i.e. whether changes in abundance and composition of the invertebrate fauna result in changes in the feeding habits of salmon parr.
20 18 Co
r'
2
16 14
I2
5 10 1 8 u
6
9
4 2 0
June
2oo
July
t
August
f"i',
c
h
I
The River Ten0 forms the border between northern Norway and Finland (70"N, 28"E), being the most productive and one of the largest salmon rivers in both countries (catchment area 16,386 km2). The River Utsjoki, a fifth-order system, is the largest tributary on the Finnish side of the drainage basin (1652 km2, Fig. 1). The second-order tributaries Vidgaveadji (1 5 km2) and Badda (34 km2) drain into the middle reaches of the River Teno, close to the outlet of the River Utsjoki (4 and 16 km from the outlet, respectively). These brooks are nursery habitats of salmon parr where salmon do not spawn but juveniles enter from the
Arch
Circ
-
40
0
April
Material and methods Study area
iso
V
May
I
June
-40
I
Iuly
August
sept
Fig. 2. Daily mean water temperatures (above) and water levels in the study rivers in 1993 (below). Water levels represent relative changes (cm) from the level in I April (=O) for Ten0 and 1 June ( = O ) for Utsjoki.
main stem (Erkinaro 1995). The River Utsjoki system is characterized by a large number of lakes: about 93% of the wetted area is comprised of lakes larger than 1 ha. In contrast, there are no lakes in the main stem of the River Ten0 or on the course of the salmon parr distribution area in the small tributaries. Physical characteristics and fish communities of the study area are described in more detail in Erkinaro ( 1 995). Water temperatures were measured during the summer months in 1993 (Fig. 2) with a Lambrecht thermograph in Utsjoki and Badah, and with a manual thermometer in Ten0 and Vidgaveadji (35 measurements per 24 h). Water levels of Ten0 and Utsjoki were recorded close to the outlet of Utsjoki. The measurements presented represent relative changes from the day when the recording started (=O). Sampling
Fig. 1. The study area
14
The sampling site on the River Ten0 main stem was located just above the confluence of the Vidgaveadji, 100 km upstream from the estuary. This part of the river is wide (>lo0 m) with open and unshaded shores. The flow velocity is usually below 0.5 m . s-l, and the substrate is predominantly pebble and cobble (Erkinaro 1995). The sampling sites in the brooks were on the lowest parts of the streams, 100-200 m above their confluences (Fig. 1). The width of the brook channels is 4-6 m, the
Feeding of salmon parr flow velocity is usually higher than in the main stem, and the substrate is coarser, consisting mainly of boulders. The brooks are mostly well shaded and partly covered by the riparian vegetation (Erkinaro 1995). In Utsjoki the sampling was focused on two habitats, the upper one (35 m wide) just below the Lake Mantojarvi (194 ha, max depth 60 mj and another 1000 m downstream (25 m wide), below a riffle section (Fig. 1). Both sites have open shadeless shoreline, flow velocity between 0.5 and 1 m.s-’, and cobble as the most characteristic substrate type. Fish (n=8-59 at each sampling site and time) were collected by electrofishing in an upstream direction to avoid catching fish that might have preyed on organisms disturbed by wading and electrofishing. Sampling took place six times during the open water season, (week/month) IV/June, I/July, III/July, I/August, III/August and III/September. In addition, some samples, IV/April and I/May in Utsjoki, and III/May in Vidgaveadji and Badda, were gathered at small open sites, while the rivers were still mostly iced over. Total length of the fish to the nearest mm was measured. The stomachs were preserved in 80% ethanol for further determination of the prey items. The relative volume of the stomach content was estimated according to a modified point scale of Hynes (1950) as: 0 (empty), 1 (
