Distinguishing Between Juvenile Anadromous and Resident Brook Trout (Salvelinus Fontinalis) Using Morphology

Environ Biol Fish DOI 10.1007/s10641-007-9186-9 FULL PAPER Distinguishing between juvenile anadromous and resident brook trout (Salvelinus fontinalis) using morphology Genevie`ve R. Morinville Æ Joseph B. Rasmussen Received: 24 May 2005 / Accepted: 17 December 2006 Ó Springer Science+Business Media B.V. 2007 Abstract Phenotypic variation linked to habitat bodies) than resident brook trout, and these use has been observed in fish, both between and differences persisted into the marine life of the within species. In many river systems, migratory fish. Migrants also exhibited shorter pectoral fins, and resident forms of salmonids coexist, including which facilitate pelagic swimming, indicating that anadromous (migrant) and resident brook trout, migrants, prior to their migration to the sea, Salvelinus fontinalis. In such populations, juvenile possess the appropriate morphology for swim- anadromous (migrant) brook trout, prior to ming in open water habitats. The reported migration, inhabit regions of higher current differences between migrants and residents were velocity than residents. Because it is more costly powerful enough to derive discriminant functions, to occupy fast currents than slow currents, differ- using only five of the seven measured traits, ences in morphology minimizing the effects of allowing for accurate classification of brook trout drag were expected between the two forms. As as either migrants or residents with an overall predicted, migrant brook trout were found to be correct classification rate of 87%. Importantly, more streamlined (narrower and shallower this study contributes to the notion that a link exists between morphology, habitat use, metabolic costs and life-history strategies. Contribution to the program of CIRSA (Centre Interuniversitaire de Recherche sur le Saumon Atlantique). Keywords Fish shape Á Habitat use Á Coexistence Á Salmonid Á Metabolic costs Á G. R. Morinville Á J. B. Rasmussen Energetics Á Sea trout Á Migration Á Anadromy Department of Biology, McGill University, 1205 Dr. Penfield Ave., Montreal, QC, Canada H3A 1B1 Introduction G. R. Morinville (&) Rescan Environmental Services, Sixth Floor, Phenotypic variation in morphological traits 1111 West Hastings, Vancouver, BC, Canada V6E 2J3 e-mail: [email protected] implicated in predator evasion, feeding and habitat use have been commonly reported in the J. B. Rasmussen literature. For example, crucian carp, Carassius Department of Biological Sciences, Water Institute carassius, increase their body depth as a defense for Semi-Arid Ecosystems (WISE), University of Lethbridge, 4401 University Dr. West, Lethbridge, mechanism against gape-limited piscivores AB, Canada T1K 3M4 (Holopainen et al. 1997; Pettersson & Bro¨ nmark 123 Environ Biol Fish 1999) and Arctic charr have evolved morpholog- had larger paired fins than those inhabiting a river ical adaptations in mouth and snout shape with lower average flows (Riddell and Leggett according to their benthivorous or planktivo- 1981). Similarly, Taylor and McPhail (1985) rous–piscivorous feeding behaviors leading to found heritable morphological differences be- four sympatric morphs in an Icelandic lake tween interior and coastal populations of juvenile (Sku´ lason et al. 1989). coho salmon. Interior populations were more Phenotypic variation linked to habitat use has streamlined in shape and possessed smaller been observed, both in warm-water stream fishes median fins than coastal populations, most likely including cyprinids and percids (Wood and Bain the result of their greater need for efficient 1995), and in salmonids, both within and between swimming during migrations, because these inte- species (Riddell and Leggett 1981; Bisson et al. rior populations originated from locations that 1988; Swain and Blair 1989). It has been consis- were further away from the sea. Interestingly, tently demonstrated that fish inhabiting faster anadromous (migrant) and non-anadromous currents are more streamlined (shallow-bodied counterparts have also been observed to differ versus deep-bodied) than those inhabiting slow morphologically such that the former is better currents. A more streamlined morphology in fast suited for prolonged swimming (Taylor and water reduces swimming costs by minimizing the McPhail 1986; Taylor and Foote 1991). Fish may effects of drag (Pettersson and Bro¨ nmark 1999). thus evolve morphological adaptions specific to Drag is influenced by fish shape, the square of the their life-history strategy and exhibit these adap- current speed and the Reynolds number (Re)of tations throughout their life stages. the fish (Vogel 1994). Re is positively related to In certain rivers, brook trout populations com- fish size, the undisturbed velocity, and water prise a mixture of anadromous (migratory) and temperature (kinematic viscosity). Pressure drag resident forms (The´riault and Dodson 2003; Mor- predominates at high velocities such that changes inville and Rasmussen 2003, 2006). It has been to a more streamlined body morphology, as suggested through various lines of support includ- indicated by the fineness ratio (ratio of standard ing bioenergetic and stable isotope analyses that in length to body depth), leads to a reduction in the such populations, juvenile anadromous (migrant) drag coefficient (Webb 1975; Blake 1983). A high brook trout, prior to migration, inhabit regions of ratio between span and length of their caudal fin higher current velocity in freshwater than resi- (‘aspect ratio’) also reduces the effects of drag dents (Morinville and Rasmusen 2003). A recent (Webb 1984, 1988). comparative study also tends to supports this—fast Using field observations, Bisson et al. (1988) habitats were occupied more frequently in streams demonstrated that differences in fin size and body containing migratory brook trout (migrant-resi- shape could predict the stream channel locations dent streams) relative to riffle habitats of streams utilized by juvenile salmonids. Coho salmon, comprising only resident trout (resident-only Oncorhynchus kisutch, prefering pools, possess a streams) (Morinville and Rasmussen 2006). deep and laterally compressed body with large In the wild, brook trout tend to swim actively median and paired fins, facilitating transitory and feed in the water column and continue to maneuvering ability. In contrast, steelhead trout, swim against the current even in fast waters Oncorhynchus mykiss, possess a fusiform body (Keenleyside 1962). As for other salmonids, trout shape with short median fins and large paired fins, occupying fast currents are expected to possess a allowing for efficient swimming in fast water, more streamlined morphology in order to mini- whereas cutthroat trout, Salmo clarki, morphol- mize the costs associated with swimming. Such ogy is intermediate to these species, consistent observations have been reported for brook trout with its habitat preferences. as early as in their first year of life (McLaughlin Within-species variations in morphology have and Grant 1994), although the differences may also been reported. In Atlantic salmon, individ- not necessarily persist over time (Imre et al. uals inhabiting a river with higher average flow 2001). Because brook trout migrate as early as velocities were more streamlined in shape and at age 1+ (The´riault and Dodson 2003), it is likely 123 Environ Biol Fish that morphological differences exist between Streams containing such populations of coexisting juvenile anadromous (prior to migration) and brook trout and Atlantic salmon will be referred resident brook trout. Distinguishing between the as ‘migrant-resident’ streams. Anadromous brook two forms at early life stages will allow for future trout down-migrate from mid-May to early-June, studies investigating coexistence and ultimately, as early as age 1+ to the estuarine Saguenay River the underlying mechanisms of anadromy. (The´riault and Dodson 2003). Stream reaches The main objectives of this study were thus to above man-made barriers, such as poorly con- (1) determine whether coexisting anadromous structed culverts and natural barriers, such as (migrant) and resident brook trout differ in waterfalls, will be referred as ‘resident-only’ body morphology, (2) whether the observed streams as these only contain resident brook differences coincide with habitat use expecta- trout. tions, and (3) whether any observed differences can be used as a tool for future field identifica- Fish collection tion. It is predicted that within migrant-resident streams (streams with resident and anadromous The genus Salvelinus exhibits the least pronounced forms), migrant brook trout will be more anadromy of salmonids (Power 1980). No obvious streamlined than residents. Specifically, it is smoltification occurs in migrant brook trout predicted that resident brook trout will have (McCormick et al. 1985) making it very difficult wider and deeper bodies (higher drag morphol- to differentiate a migrant from a resident until the ogy) compared to migrant brook trout. Fin sizes moment of migration. In migrant-resident streams, including caudal, pelvic and pectoral are also we distinguished migrants as trout captured in trap expected to differ between migrants and nets during the downstream migration period, residents. Furthermore, it is predicted that whereas those captured in streams following the resident fish from migrant-resident streams will migration period were defined as residents. Tag- be less streamlined than those of resident-only ging studies confirmed that this method was effec- streams

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