Transferability of habitat suitability criteria of juvenile Atlantic salmon ( Salmo salar )

Transferability of habitat suitability criteria ofjuvenile Atlantic salmon (Salmo salar)

Aki Mäki-Petäys, Ari Huusko, Jaakko Erkinaro, and Timo Muotka

Abstract: We constructed generalized habitat criteria for juvenile Atlantic salmon (Salmo salar) based on four river-specific suitability indices for depth, water velocity, and substrate to assess whether habitat criteria for juvenile Atlanticsalmon are transferable across rivers. We first tested whether salmon are more likely to occupy higher-quality habitatsthan generally available in a stream reach based on these composite criteria. We then repeated the same procedureusing the river-specific criteria of this study and the generalized habitat suitability criteria of Heggenes. As expected,the river-specific criteria were generally the most effective ones in predicting fish habitat use. However, both of the twogeneralized criteria also transferred fairly well to the test sites. Viewed across salmon size-classes (<9 cm and >9 cm),the river-specific criteria passed the test in ten of eleven cases (91%), and the two generalized criteria passed in nine(82%) of eleven. Thus, it appears that with respect to summertime habitat criteria for juvenile Atlantic salmon, criteriatransference is conceivable at least on a regional scale, and perhaps even on a more “universal” scale.

Résumé : Nous avons élaboré des critères généraux pour évaluer l’habitat de jeunes saumons de l’Atlantique (Salmosalar) d’après quatre indices de qualité spécifiques à des cours d’eau particuliers et basés sur la profondeur de l’eau, lavitesse du courant et le substrat dans le but de déterminer si ces critères pouvaient s’appliquer à d’autres rivières. Nousavons d’abord vérifié si les saumons étaient plus susceptibles de se retrouver dans les habitats de qualité supérieuredéfinis par ces critères composites que dans les habitats généralement disponibles dans la section de rivière. Nousavons répété l’opération à l’aide de des indices spécifiques à une rivière et des critères généraux de qualité de l’habitatde Heggenes. Tel que prévu, les indices spécifiques étaient les plus efficaces pour prédire l’utilisation de l’habitat parles poissons. Cependant, les deux séries de critères généraux convenaient assez bien aux sites expérimentaux.Appliqués aux classes de taille de saumons (<9 cm et >9 cm), les critères spécifiques aux rivières se sont avérésadéquats dans dix (91 %) des onze cas et les deux séries de critères généraux dans neuf (82 %) cas. Il apparaît doncque l’on puisse transférer d’un cours d’eau à l’autre les critères de qualité de l’habitat des jeunes saumons del’Atlantique en été, au moins dans une même région et peut-être même à une échelle plus « universelle ».

[Traduit par la Rédaction] Mäki-Petäys et al. 228

Introduction

Habitat selection is an important component of animaldistribution, linking together behavioural ecology and popu-lation ecology (Kramer et al. 1997). For stream fishes, habi-tat selection is often related to trade-offs among swimmingcosts, prey capture success, and risk from predators oradverse environmental conditions (Hughes and Dill 1990;Heggenes et al. 1993). The relative importance of varioushabitat factors influencing fish distribution and abundancedepends upon the scale at which they are examined. Viewedacross stream reaches, some species or size- classes may bepool or riffle dwellers (Angermeier and Karr 1983), whereasat the within-reach scale, depth, water velocity, substrate,

and cover are commonly considered as the main variablesaffecting the distribution of fishes (e.g., Heggenes 1990).Suitability criteria for these instream variables at differentlife stages of fish are used as biological components in habi-tat hydraulic models (e.g., physical habitat simulation,PHABSIM; Bovee 1982) to estimate the quantity of habitatsuitable for fish at different flows.

Early attempts to model fish habitats in streams assumedthat habitat requirements of fish are “universal” (Morantz etal. 1987). This assumption implies that fish prefer similarhabitat conditions regardless of habitat availability, whichwould make site-specific habitat studies unnecessary. Thisnotion has been invalidated, however, in many studies em-phasizing the influence of habitat availability on habitat use

Can. J. Fish. Aquat. Sci. 59: 218–228 (2002) DOI: 10.1139/F01-209 © 2002 NRC Canada

218

Received 24 April 2001. Accepted 7 December 2001. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on15 February 2002.J16320

A. Mäki-Petäys1 and J. Erkinaro. Finnish Game and Fisheries Research Institute, Oulu Game and Fisheries Research,FIN-90570 Oulu, Finland.A. Huusko. Finnish Game and Fisheries Research Institute, Kainuu Fisheries Research and Aquaculture, FIN-88300 Paltamo,Finland.T. Muotka. Department of Biological and Environmental Science, University of Jyväskylä, P.O. Box 35, 40351 Jyväskylä, Finland.

1Corresponding author (e-mail: [email protected]).

J:\cjfas\cjfas59\cjfas-02\F01-209.vpWednesday, February 13, 2002 8:05:54 AM

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by fish (e.g., Heggenes 1990). In addition, habitat use maybe affected by other factors, such as food availability, com-petition, predation (e.g., Orth 1987), water temperature(Mäki-Petäys et al. 1997), time of day (Heggenes et al.1993), and discharge (Pert and Erman 1994). Therefore, manyauthors have cautioned against generalizations based on sin-gle “snapshot” studies and have recommended the use ofsite-specific habitat criteria whenever possible (e.g., Moyleand Baltz 1985; Greenberg et al. 1996). Nevertheless, fisher-ies managers are often required to make prompt decisionsregarding stream habitat enhancement and protection, and itis not logistically and economically feasible to develop habi-tat criteria for all environmental conditions that may influ-ence fish habitat. Therefore, it is a common practice to applyhabitat criteria to streams other than those for which theywere developed (Armour and Taylor 1991). Notwithstandingthe increasing need for habitat hydraulic models in streammanagement, only a few tests of the transferability of habitatcriteria have been performed (but see Thomas and Bovee1993; Groshens and Orth 1994; Freeman et al. 1997). Suchtests are needed, however, because these criteria constitute amajor component of habitat hydraulic models and shouldtherefore reliably reflect the habitat requirements of a targetspecies. One approach for conducting such tests is to assesswhether fish habitats predicted by the criteria constitute asubset of higher-quality habitats than generally available in astream.

In the present study, we examined whether habitat criteriadeveloped for juvenile Atlantic salmon (Salmo salar) aretransferable across rivers. Heggenes (1990) suggested thatstudies on habitat use by salmon would be most rewardingwhen conducted in streams with widely differing habitatavailability. We examined the summer habitat use by juve-nile salmon in four geographically distinct Finnish riverssupporting genetically separate populations of Atlanticsalmon (Nilsson et al. 2001). These rivers form a continuumfrom a naturally reproducing salmon population to a popula-tion maintained through regular stocking, and their fishassemblages show variation from a salmon-dominated as-semblage to a diverse fish community. For each of theserivers, we developed suitability curves for two size-classesof salmon using three habitat variables: depth, water veloc-ity, and substrate. We also constructed composite suitabilitycurves based on the four river-specific curves to avoid biasescaused by snapshot studies in one place and time. Finally,we assessed the transferability of these generalized curves intwo ways. First, we compared the curves developed in thisstudy with the generalized habitat curves for juvenile Atlan-tic salmon constructed by Heggenes (1990). Then, we testedwhether juvenile salmon are more likely to occupy higher-quality habitats than generally available in the study sectionbased on (i) river-specific criteria, (ii) generalized criteria ofthis study, and (iii) generalized criteria of Heggenes (1990).

Materials and methods

Study areasThis study was performed in four Finnish rivers: Tenojoki,

Simojoki, Pyhäjoki, and Koitajoki (Fig. 1). The largest andnorthernmost of these is River Tenojoki (70°N, 28°E, mean

flow 160 m3·s–1), which flows through subarctic terrain anddrains into the Arctic Ocean. It has extremely clear waterwith low concentrations of nutrients (Table 1). The riversSimojoki (66°N, 25°E, mean flow 38 m3·s–1) and Pyhäjoki(64°N, 25°E, mean flow 29 m3·s–1) discharge into theBothnian Bay, Baltic Sea. River Koitajoki (63°N, 30°E,mean flow 2 m3·s–1) is a second-order inland river discharg-ing into River Pielisjoki. Koitajoki has a fixed regulatedflow because of the diversion of the main flow from theoriginal route for power production. These three rivers runthrough boreal coniferous forests and agricultural landscapesand are characterized by brown waters and higher nutrientconcentrations (Table 1).

Given the size of the river, an annual catch of 100–200tonnes (t) of salmon, and high juvenile salmon densities,River Tenojoki supports probably the largest wild Atlanticsalmon population in the world (Niemelä et al. 1999). Theoccurrence of other fish species in the shallow fluvial habi-tats of this river is negligible (Table 1).

In River Simojoki, wild Baltic salmon stocks decreaseddramatically during the latter half of the 20th century, but re-cent efforts to enhance the remaining stocks have been suc-cessful: densities of wild juvenile salmon in River Simojokiincreased substantially in the late 1990s, approaching thosein River Tenojoki. A salmon stock rebuilding programme inRiver Pyhäjoki started in the late 1990s, and the first signsof wild salmon reproduction have been detected recently.However, a great majority of juvenile salmon in RiverPyhäjoki is still of hatchery origin. Fishes other than salmonconstitute a considerable proportion of the fish assemblagein the two Baltic rivers Simojoki and Pyhäjoki (Table 1).

The land-locked salmon stock of River Koitajoki declinedin the early 1960s when hydropower development shut offthe migration route for spawners, thus preventing natural re-production. Nowadays the salmon population is maintainedthrough regular stocking of salmon parr. Fish species otherthan salmon constitute a considerable proportion of the fishspecies assemblage in River Koitajoki. High density ofbrown trout (Salmo trutta) is the main difference comparedwith other study rivers (Table 1).

Field protocolsHabitat use by juvenile salmon was studied in July–

September 1994 (Tenojoki), 1997 (Pyhäjoki), and 1999(Koitajoki and Simojoki). Water temperature exceeded 10°Cin all study rivers during these months, which facilitatesamong-river comparison of fish habitat use because an au-tumnal shift in habitat use and behaviour of salmonids is re-ported to occur at water temperatures below +10°C (e.g.,Cunjak 1988; Heggenes et al. 1993).

According to Heggenes (1990), no method for locatingfish (surface observation, diving, electrofishing) works wellin all habitat conditions. Therefore, we chose to use both un-derwater observation and electrofishing in this study. Under-water observation was the most appropriate method in RiverTenojoki, where water visibility is good and the study sitesare rather deep (see Table 1). Because of lower water visibil-ity (<3 m) and shallower stream habitats, electrofishing wasused in other rivers. Obviously, these differences in fieldmethods may cause some apparent differences in fish habitat

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220 Can. J. Fish. Aquat. Sci. Vol. 59, 2002

use between River Tenojoki and the other rivers, becauseunderwater observation misses fish in shallow (<20 cm) wa-ter and electrofishing misses fish in deep waters (Heggenes1990).

Underwater observations were made with a mask and snor-kel using methods described by Heggenes (1991). A divermoved systematically upstream in a zigzag fashion to ran-domize any bias associated with nearshore or offshore obser-vations. Once an undisturbed fish was located, fish lengthwas estimated and assigned to one of two size-classes,<9 cm or >9 cm. These size-classes correspond roughly toage-0 and age-1 fish, respectively, except for River Tenojoki,where juvenile salmon grow more slowly (see Erkinaro andNiemelä 1995) and, therefore, fish smaller than 9 cm in-cluded many age-1 fish.

A point electrofishing procedure was used to reduce the

potential fright bias caused by electrofishing (e.g., Moyleand Baltz 1985; Heggenes 1991). The anode was submersedat equidistant points (0.5 m) along transects spaced 1 mapart along the stream section to be sampled. Backpackelectroshocker with low voltage (350 V) and a 10-cm-diameter anode was used to reduce the positive galvano-taksis induced by DC electroshockers (Copp 1989). Fish po-sition was ascribed to the point of first sighting of a fish.Salmon captured were measured to the nearest millimetre.

Depth of water column, mean water velocity, and substratumparticle size were recorded for each fish position. Mean watervelocities were measured at 0.6 × depth with a SchiltknechtMiniAir2 flow meter (Schiltknecht Messtechnik AG, Gossau,Switzerland) fitted with a 20-mm propeller. Dominant sub-strate was determined visually in 0.8 × 0.8 m quadrats di-rectly below the fish. Substrate particle size was classified

Fig.1. Location of the study areas and study reaches (�) in rivers Koitajoki (63°N, 30°E), Pyhäjoki (64°N, 25°E), Simojoki (66°N,25°E), and Tenojoki (70°N, 28°E) in Finland.



according to a modified Wentworth scale, using categoriesfrom sand to bedrock in Heggenes’ (1991) classification(<5, <32 mm; 5, 32.1–64 mm; 6, 64.1–128 mm; 7, 128.1–256 mm; 8, 256.1–512 mm; 9, 512.1–1024 mm; 10, bed-rock). To quantify habitat availability at each study site, thesame variables were measured at 1.0-m intervals (0.5 m inRiver Tenojoki) along 5–30 equidistant transects perpendic-ular to the flow. Because of variable fish density, the areasampled varied between 100 m2 and 2500 m2, resulting in1056, 648, 1313, and 711 measurement points in riversKoitajoki, Pyhäjoki, Simojoki, and Tenojoki, respectively(see Table 2).

Data analysisHabitat selection of fish is based on concurrent evaluation

of several interacting habitat characteristics (Shirvell andDungey 1983; Heggenes 1991). Multivariate analyses aresuccessful in addressing such problems, because they sum-marize multivariate information into a few ecologicallyinterpretable gradients (e.g., Grossman and Freeman 1987;Mäki-Petäys et al. 1997). Accordingly, we used principalcomponent analysis (PCA) to examine among-river differ-ences in habitats available for and used by juvenile salmon.Data from systematic sampling points, as well as those usedby fish, in each study river were pooled for this analysis.Among-river differences in PCA scores of habitats availableand used by fish along the first PCA axis were evaluated byone-way analysis of variance (ANOVA), followed byTukey’s test for pairwise comparisons.

We developed river-specific habitat suitability criteria(hereafter HC1) by constructing suitability curves for eachvariable using the method described by Baltz (1990). Habitatsuitabilities were defined as (proportional use)/(proportionalavailability) and they were standardized to range from 0.0(unsuitable) to 1.0 (optimal). A smoothed suitability curvewas fitted to the data using the distance weighted least

squares (DWLS) smoothing option in SYSTAT (SYSTAT1992). We then constructed “general” suitability curves foreach habitat variable (hereafter HC2) by using the arithmeticmeans of the river-specific suitability indices for each incre-ment. This should mitigate any bias caused by snapshotstudies at one place and time and should give a more generalview of the habitat requirements of juvenile salmon(Heggenes et al. 1996).

We then estimated the transferability of the HC2 curves intwo ways. First, we compared our HC2 curves with thegeneralized habitat suitability curves for juvenile Atlanticsalmon constructed by Heggenes (1990) (hereafter HC3).For this purpose, the HC3 curves were also standardized torange from 0.0 to 1.0.

Secondly, based on the HC2 criteria, we examined whetherjuvenile salmon are more likely to occupy high-quality habi-tats than habitats classified as suboptimal. For this purpose,we used a method based on Thomas and Bovee (1993), withsome modifications. We first converted habitat variables foreach sampling point to a range of 0.0 to 1.0, using the HC2curves for depth, water velocity, and dominant substrate. Wethen calculated the geometric mean of the suitability indexof the three variables to form a single composite index foreach sample point. The use of joint-suitability indices hasbeen criticized (e.g., Heggenes et al. 1996), because they as-sume that fish select each habitat variable independently ofother variables. However, Vadas and Orth (2001) have re-cently shown that such aggregation of suitability indices forindividual habitat variables can reliably be used to predictfish density in streams. Habitats with a combined suitabilityvalue ≥ 0.75 were defined as optimal, whereas habitats withan index value between 0.50 and 0.75 were defined as suit-able, and those less than 0.50 were defined as unsuitable forjuvenile salmon of each size-class. Thomas and Bovee(1993) showed that with less than 55 occupied cells, the in-cidence of type I, and especially of type II, errors increases

© 2002 NRC Canada


River Koitajoki River Pyhäjoki River Simojoki River Tenojoki

GeneralLocation 63°N, 30°E 64°N, 25°E 66°N, 25°E 70°N, 28°ECatchment area (km2) 6795 3711 3160 16 386Annual mean flow (m3·s–1)a 2a 29 38 160Water qualityb

pH 6.5–7.0 6.5–7.0 6.5–7.0 6.8–7.9Alkalinity (meq·L–1) 0.08–0.1 0.1–0.3 0.1–0.3 0.2–0.4Conductivity (mS·m–1) 2.1–3.0 4.0–22.0 2.1–6.5 3.2–8.0Total P (mg·L–1) 10–15 40–50 17–22 3–11Colour (mg Pt·L–1) 80–120 140–200 90–140 15–20Fish densityc (individuals/100 m2)Atlantic salmon (Salmo salar) 26–86 2–47 42–117 26–160Brown trout (Salmo trutta) 61–156 0–3 0 0Sculpins (Cottus sp.) 27–100 5–83 14–42 0–2Grayling (Thymallus thymallus) 0 0–3 2–15 0Minnow (Phoxinus phoxinus) 0 0–8 2–45 0Burbot (Lota lota) 28–100 0–2 1–2 0Stone loach (Noemacheilus barbatulus) 31–71 27–113 16–34 0

aBased on at least 20 years of daily flow measurements.bRegulated fixed flow, during short periods (<1 month annually) discharge exceeds up to 100 m3·s–1.cRange among study sites.

Table 1. Main characteristics of the study rivers.



dramatically. Because such a large sample size of occupiedhabitats was obtained only rarely in our study, we relaxedthis assumption somewhat and accepted data with more than30 observations for habitat occupied by fish, provided thatthe number of measurements for habitat availability was suf-ficiently large (>200; see Thomas and Bovee 1993). Ac-cordingly, transferability tests were performed in five studysites for salmon <9 cm (one site in rivers Pyhäjoki,Tenojoki, and Koitajoki each, two in River Simojoki) and insix sites for salmon >9 cm (three sites in River Simojoki,two in Koitajoki, one in Pyhäjoki). A two-tailed χ2 test wasused to test for nonrandom habitat selection by fish. Thesame procedure was repeated using the HC1 and HC3curves to compare the transferability of the HC2 curves withboth more site-specific (HC1) and more general (HC3) habi-tat criteria.

Results

PCA on habitat use and availabilityPCA on habitat availability and use (data pooled across

study rivers) extracted one component with eigenvalue > 1.0(42.0% of variance explained; Fig. 2). This component rep-resented a gradient from shallow, low-velocity habitats withcoarse substrates to deep, swift currents with finer substrates.ANOVA indicated significant river-specific differences inhabitat use by juvenile salmon, but this mainly reflected aparallel change in habitat availability (see also Table 2). Allpairwise differences in habitat availability between the studyrivers were significant (Tukey’s test, p < 0.05; see Fig. 2a).The deepest habitats were found in River Tenojoki, and theshallowest ones were found in River Koitajoki (Table 2).Habitat use by small salmon (<9 cm) largely paralleled vari-ation in availability, with all pairwise differences being sig-nificant except that between rivers Koitajoki and Pyhäjoki(Tukey’s test, p > 0.05). For larger salmon (>9 cm), onlyhabitats used in River Tenojoki differed significantly (deeperwater) from those used in other rivers.

Habitat suitability criteria of juvenile Atlantic salmonWide tolerance of various habitat conditions by juvenile

salmon was well indicated by their habitat suitability curves(Figs. 3a and 3b). In particular, both size-classes seemedrather indifferent with respect to mean water velocity, be-cause optimum velocities varied from 20 to 80 cm·s–1. RiverTenojoki excluded, the among-river variation in optimumdepths varied from 15 to 60 cm and from 25 to 65 cm forsmall and large juvenile salmon, respectively. In Tenojoki,optimal habitats were at about 20 cm deeper stream areas(Fig. 3). Cobble-to-boulder substrates were used selectivelyby both size-classes in all four rivers, though smaller salmonpreferred slightly finer substrates. The apparent avoidance of

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RiverWatertemperature (°C)

Depth(cm)

Mean watervelocity (cm·s–1)

Dominantsubstrate (class) N

Koitajoki Availability 14–20 27 ± 17.2 28 ± 27.5 7.9 ± 1.2 1056<9 cm 29 ± 10.5 33 ± 26.8 7.8 ± 0.9 137>9 cm 33 ± 13.5 40 ± 29.4 8.0 ± 0.9 169

Pyhäjoki Availability 13–17 31 ± 13.3 30 ± 16.2 7.5 ± 1.2 648<9 cm 33 ± 7.6 32 ± 16.1 7.3 ± 1.2 83>9 cm 34 ± 10.1 28 ± 14.5 7.7 ± 1.2 124

Simojoki Availability 12–16 37 ± 20.4 39 ± 31.0 7.4 ± 2.1 1313<9 cm 37 ± 15.1 40 ± 27.9 6.7 ± 2.0 229>9 cm 38 ± 15.2 35 ± 28.5 7.9 ± 1.6 135

Tenojoki Availability 11–15 47 ± 21.8 33 ± 18.2 6.7 ± 1.1 711<9 cm 52 ± 19.2 32 ± 11.8 6.6 ± 0.7 79>9 cm 56 ± 17.4 34 ± 15.7 7.1 ± 1.1 59

Note: Means ± standard deviations are shown for depth, mean water velocity, and dominant substrate size. N = number of measurements. Substrate isclassified according to the Wentworth scale (modified from Heggenes 1991): 1, 0.07–2 mm; 2, 2.1–8 mm; 3, 8.1–16 mm; 4, 16.1–32 mm; 5, 32.1–64 mm; 6,64.1–128 mm; 7, 128.1–256 mm; 8, 256.1–512 mm; 9, 512.1–1024 mm; 10, bedrock.

Table 2. Habitats available for and used by two size-classes (<9 cm and >9 cm) of juvenile salmon in the study rivers.

Fig. 2. Principal component analysis (PCA) on among-river dif-ferences in habitat availability and use by juvenile Atlanticsalmon. Mean PCA scores, with 95% confidence intervals, alongthe first component are presented for (a) habitat availability anduse by (b) salmon <9 cm and (c) salmon >9 cm. Rivers (�,Koitajoki; �, Pyhäjoki; �, Simojoki; �, Tenojoki) sharing a let-ter do not differ significantly (ANOVA followed by Tukey’spairwise comparisons). Loadings for the original variables alongthe first PCA axis are also given.



large-boulder areas in River Tenojoki was mainly caused bythe of lack such substrates in this river.

Transferability of habitat criteriaDespite their different origins, both generalized habitat

criteria (HC2 vs. HC3) indicated surprisingly similar habitat

suitabilities by both size-classes of juvenile salmon (Figs. 4aand 4b). However, the HC2 curves indicated tolerance of upto 20 cm·s–1 swifter currents than the HC3 curves. Accord-ing to HC2, the ranges of optimum velocities were 15–60 cm·s–1 and 15–70 cm·s–1 for small and large juvenilesalmon, respectively. Depth and substrate curves showed

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Fig. 3. Suitability curves for depth, mean water velocity, and dominant substrate for juvenile Atlantic salmon (a) <9 cm and (b) >9 cmin the study rivers (—, Koitajoki; — —, Pyhäjoki; – –, Simojoki; ....., Tenojoki).

Fig. 4. Generalized, across-river habitat suitability criteria of this study (HC2, solid line) and generalized criteria modified fromHeggenes (1990; HC3, dotted line) for depth, mean water velocity, and dominant substrate for juvenile Atlantic salmon (a) <9 cm and(b) >9 cm. HC3 curves are standardized to range from 0 to 1. Arrows indicate the lower and upper limits of the optimal suitabilities(index values > 0.75) for the HC2 criteria, based on the four river-specific (HC1) criteria (means ± standard error). Note thatHeggenes’ size-class separation (<7 cm for age-0, >7 cm for parr) differs slightly from that used in this study.



© 2002 NRC Canada


similar size-related changes in habitat suitability. The rangeof optimum depth for HC2 curves was 25–55 cm and 25–60 cm for smaller and larger salmon, respectively. The mostdistinct difference between the two types of criteria was thatthe HC2 substrate curves were more biased towards largeparticles, indicating preference for these particle sizes by ju-venile salmon. Using HC2 curves, the optimal substrate sizevaried from large pebbles to boulders for salmon <9 cm and

from cobbles to large boulders for salmon >9 cm.For smaller salmon, both HC1 and HC3 criteria trans-

ferred well to all five test sites, and HC2 criteria transferredwell to four of the five test sites (Fig. 5). For larger salmon,HC1 and HC2 criteria had successful transference to fivetest sites, and HC3 criteria had successful transference tofour of the six test sites (Fig. 6). The river-specific criteria(HC1) were, however, overall the most effective ones in pre-

Fig. 5. The quality of habitat available for (open bars) and used (shaded bars) by salmon <9 cm in five study reaches (see Fig. 1 for location ofthe sites) based on river-specific habitat suitability criteria (HC1), generalized criteria of the present study (HC2), and generalized suitability crite-ria of Heggenes (1990; HC3). Asterisks indicate results from a two-sided χ2 transferability tests: ***, p < 0. 001; **, p < 0.01; *, p < 0.05; (*),p < 0.10; ns, nonsignificant.



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Fig. 6. The quality of habitat available for (open bars) and used (shaded bars) by salmon >9 cm in six study reaches (see Fig. 1 forlocation of the sites) based on river-specific habitat suitability criteria (HC1), generalized criteria of the present study (HC2), and gen-eralized criteria of Heggenes (1990; HC3). For other details, see Fig. 5.



dicting fish habitat use. For one site (S3 for larger salmon),all transferability tests failed; this site had a low (but not thelowest) sample size for habitats occupied by fish (n = 40;see Fig. 6).

Discussion

Rivers of this study are geographically distinct, represent-ing a gradient of contrasting habitat availability for juvenilesalmon. River Tenojoki is a near-pristine river and is one ofthe northernmost salmon rivers in the world. By contrast, theother study rivers, located 500–900 km southward fromRiver Tenojoki, are variously affected by human activities(forestry, agriculture, power production). Furthermore, therivers harbour widely differing fish assemblages, with poten-tially large variation in the importance of biotic interactionson fish habitat selection. Despite such wide heterogeneityamong the rivers, our results on habitat use by different-sized juvenile Atlantic salmon were rather consistent andwell in accordance with previous findings (see Heggenes1990; Heggenes et al. 1996, 1999). Habitat selection byjuvenile salmon was clearly size-structured, larger fish pre-ferring deeper, faster-flowing water with coarser substrates.Our results also demonstrated that river-specific differencesin habitat use and suitability by salmon mainly reflect a con-current change in habitat availability among the rivers (seealso Heggenes 1991).

Thomas and Bovee (1993) argued that the most significantsource of error in habitat hydraulic modeling is the selectionof habitat criteria applied. Many authors have suggested thatbecause so many factors influencing fish habitat use vary lo-cally, habitat criteria should be developed in situ instead ofusing universal criteria or transferring criteria amongstreams (e.g., Moyle and Baltz 1985; Heggenes 1990). This,however, is an unattainable goal, and the often uncritical ac-ceptance of criteria developed in another stream will likelyremain a prevailing option in many, if not most, modelingapplications. The generalized criteria for juvenile Atlanticsalmon developed in this study (HC2 curves) converged tothe generalized suitability criteria of Heggenes (1990) (HC3curves), suggesting that summertime habitat requirements ofjuvenile Atlantic salmon are relatively invariable. However,a mere visual inspection of suitability curves gives little ba-sis for arguments about criteria transferability. Transferabil-ity tests developed by Thomas and Bovee (1993) represent astep forward, although they were criticized by Williams etal. (1999) as being generally weak and of limited biologicalvalue. Obviously, a more comprehensive test would linkhabitat criteria with hydraulic models to predict fish distribu-tions in a test riffle at various discharges (e.g., Guay et al.2000), but this was not the goal of our study. Instead, wewanted to test directly the transferability of habitat criteria,which themselves constitute a major component of habitathydraulic models. We thus concur with Freeman et al.(1999) in that transferability tests do provide biologicallyvaluable information with obvious importance to streammanagers, albeit inefficient to some degree. This is becausethey assess whether fish habitats predicted by these criteriaconstitute a subset of higher-quality habitats than generallyavailable in a stream, and by doing so, they test whether theinput variables (depth, velocity, substrate) are able to capture

the key elements of habitats required by a target species.Such information on the predictability of fish habitat re-quirements is of considerable practical value in stream fish-eries management, regardless of its role in modelingapplications (e.g., Mäki-Petäys et al. 1999).

In this study, both the river-specific (HC1) and the gener-alized (HC2 and HC3) habitat criteria transferred fairly wellto test sites. Viewed across salmon size-classes, the HC1 cri-teria passed the test in ten of eleven cases (91%), and theHC2 and HC3 criteria passed in nine of eleven cases (82%).Thus, with respect to summertime habitat criteria for juve-nile Atlantic salmon, it appears that criteria transference isconceivable, at least on a regional scale (see Greenberg et al.1996), and perhaps even on a more universal scale. It couldeven be argued that considering the spatially and temporallydynamic nature of habitat selection by fish, the use of gener-alized criteria might be advisable in most modeling contexts.This is because site-specific criteria are commonly based onsnapshot studies covering only a fragment of the range ofhabitats suitable for salmon (e.g., Heggenes et al. 1996).Furthermore, although site-specific criteria are bound to pro-vide the best fit to site-specific data on fish habitat use, therationale behind this approach is somewhat circular: dataused to develop suitability criteria are also used to predictsuitability.

The generally good transferability of habitat criteria in ourstudy is in accordance with the suggestion by Freeman et al.(1997) that criteria for riffle-dwelling fishes provide moretransferable measures of habitat quality than those for spe-cies mostly found in deeper habitats. Juvenile salmon mostlyoccupy riffle habitats with swift currents, and deep pools areless typical for their residence (e.g., Heggenes et al. 1999;but see Bremset and Berg 1997). The good transferability ofthe criteria indicates that they reflect rather well the habitatconditions that best support the survival and growth ofsalmon parr in the test sites (see Freeman et al. 1997).Transference would probably have been even better if habitatconditions in the test sites had been more diverse. Examina-tion of the overall quality of the test sites based on thesecriteria showed that most habitats available in these sitesrepresented nearly optimal habitat conditions for both size-classes of salmon.

It is important to realize, however, that fish abundancemay be limited by habitat suitable for a previous life stage asin the population under study (Fausch et al. 1988; Mäki-Petäys et al. 1999). In boreal streams, lack of suitable over-wintering areas and sedimentation of salmon spawninggrounds by humic substances are obvious candidates for fac-tors that operate beyond the seasonal and (or) life stage con-straints of traditional fish habitat studies (Mäki-Petäys et al.1999; Laine and Heikkinen 2000; Solazzi et al. 2000).Therefore, to increase the applicability of habitat modelingin stream fisheries management, future studies on habitat useby juvenile salmonids should identify and test the generalityof the key factors underlying the most limiting “bottleneck”stages during the life cycle of a fish.

Acknowledgments

We thank Olli van der Meer, Petri Kreivi, Alpo Huhmarniemi,Ari Kauttu, Igor Schurov, and Slava Maslov for conducting the

© 2002 NRC Canada




© 2002 NRC Canada


fieldwork for this study. Eero Niemelä is acknowledged for lo-gistical help during the fieldwork in River Tenojoki. We warmlyacknowledge the critical review by Leo Kuikka on an earlierversion of the manuscript.

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Transferability of habitat suitability criteria of juvenile Atlantic salmon ( Salmo salar )

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