**4. Discussion**

344 Modern Telemetry

**+34164.7\*x-492.9\*x<sup>2</sup>**

**-4812.0\*x+71.6\*x<sup>2</sup>**

**+5218.1\*x-77.5\*x<sup>2</sup>**

**0.01-3.00h 3.01-6.00h 6.01-9.00h 9.01-12.00h 12.01-15.00h 15.01-18.00h 18.01-21.00h 21.00-24.00h Period (hours)**

Fig. 12. Activity pattern based on polynomial regressions, performed for native trout, considering the different size classes: A) small < 15.0 cm; B) median 15.0-20.0 cm; C) big > 20.0 cm) using PIT Telemetry technology relative to eight diel periods and three hours classes in the Baceiro stream, during summer 2005. The dependent variable represents the

The comparisons between polynomial regressions calculated for the native size classes and stocked brown trout (Figures 12 and 13), showed similar behaviour only for the bigger individuals (dominant native and stocked trout) in spite of the increased probability of spatial competition and agonistic events. However, the morphological (fin deformities, hyperbuoyancy), physiological (stress response) and behavioural (lack of social hierarchy, weak territorial behaviour) characteristics presented by many stocked trout could explain their potentially disadvantageous performance in relation to dominant wild fish. The higher density referred for the PIT experiment established, did not affect the body condition of dominant native trout and contribute to explain the superior capacity to explore the available resources, namely in terms of feeding and resting activities. This pattern was not observed for smaller native trout and for stocked trout populations that showed a significant decrease in the body condition during the five

relative probability of use (standardized to a 0-1 scale).

**+3.16\*x<sup>3</sup>**

**+0.5\*x<sup>3</sup>**

**-0.5\*x<sup>3</sup>**

**-0.008\*x<sup>4</sup>**

**-0.001\*x<sup>4</sup>**

**+0.001\*x<sup>4</sup>**

**Small native = -8.9\*10<sup>5</sup>**

**0.0**

weeks experiment.

**0.2**

**0.4**

**Relative Probability of Use**

**0.6**

**0.8**

**1.0**

 **Small native Median Native Big native**

**Median Native = -1.3\*10<sup>5</sup>**

**Big native = 1.2\*10<sup>5</sup>**

Native brown trout showed a significantly less dispersal behaviour than stocked trout. However, caution should therefore be taken in the interpretation of this result since only two fish were considered in the exploratory experiment. Nevertheless, the limited movement exhibited by native fish was also identified in several studies (Knouft & Jutila, 2002; Maia, 2003), although, as referred by Bunnel et al. (1998), it may be function of the size of the habitat that provides adequate feeding and resting zones. These authors mentioned that brown trout movement varied among individuals of a same population, but most fish moved within a single continuous riffle/run-pool sequence in a diel cycle. In opposition to the resident trout movement, strictly related with the energetic cost/benefit ratio, Bachman (1984) found an erratic behaviour for the recently stocked trout. According to this study, confirmed by personal observations, hatchery-reared brown trout displayed a typical behaviour acquired in raceway tanks and moved constantly, leading to an excessive expenditure of energy in the swimming activity and agonistic encounters, which contributed to poor growth and survival rates in the wild environment. Stocked radiotagged trout in the Baceiro stream showed a clear tendency to dispersal in downstream

Combining Radio and PIT-Telemetry to Study the Large and Fine-Scale

banks suggested the higher mortality detected, mainly on stocked fish.

Movements of Stocked and Wild Brown Trout (*Salmo trutta* L.) in a Northeastern Stream, Portugal 347

2003; Aarestrup et al., 2005). The significant decrease in the condition of stocked brown trout suggested a lower ability to explore the available resources. In fact, the inefficient behaviours displayed by hatchery-reared fish contributed to a lesser adaptation to wild environment when compared with native trout. For example, higher aggression levels (Deverill et al., 1999), lack of social dominance structures (Jenkins, 1971), lower efficiency at feeding on wild prey (Olla et al., 1994), higher metabolic rates (Ersbak & Haase, 1983) and reduced swimming ability are reported for stocked salmonids. On the other hand, the hatchery-reared fish are more vulnerable to angling and natural predation ((Ludwig et al., 2002; Jacobsen, 2005) and higher mortality rates are associated near large stocked fish releases (Marnell, 1985). Observations from the stream bank revealed, during the experimental periods, a reduced fright response of stocked trout to human presence, confirming their potential weakness to avoid natural predation. In fact, most of the Portuguese northeastern salmonid streams supported growing populations of otter (*Lutra lutra* L.) and the higher number of spraints (4 PIT tags were detected) observed on stream

Stocked trout movements, for larger and even for smaller PIT tagged fish, were greater than for every native trout class defined. These fishes displayed an activity pattern more intense during day-light hours, namely during the dawn period. Normally, hatchery-reared trout are more active than wild fish (McLaren, 1979) and the higher mobility pattern showed during day-light hours in this study was, probably, related to the rearing environment (feeding habits in the hatchery) and the increasing ability of fish to detect food as light intensity increases (Fraser & Metcalfe, 1997). Furthermore, these results are according to the exploratory experiments made over two weeks in the previous year and confirmed the lack of a capacity to define a territory and a non-cost-effective behaviour, also detected by other authors (Bachman, 1984). In fact, the importance of habitat variables for both populations was similar during the 5 weeks suggesting that the stocked fish did not change their strategy in terms of habitat use. Hatchery-reared fish occupied habitats away from the stream banks, mainly, without aquatic cover (*e.g.* boulders, roots) and overhanging vegetation (shading). Other studies confirmed the distinct behaviour between wild and stocked salmonids based on the referred variables (Magoulick & Wilzback, 1997). Obviously, the different habitat use and the less concealment behaviour (Bachman, 1984) of stocked trout relative to wild fish increase their visibility and the vulnerability to avian and aquatic predators. A low influence of PIT tag surgical implantation in the fish peritoneal cavity was reported on different studies, with regard to survival, growth, swimming performance and general behaviour (Riley et al. 2003). During this study, trout were recaptured five weeks after their release in the stream, and healing was completed, without signs of inflammation or necrosis in the tissues, suggesting that the behaviour of both sympatric populations was not affected by the tagging procedures. Furthermore, only two stocked fish presented signs of tag expulsion, in spite of the incisions not having been closed with sutures for both 12 and 34 mm PIT tags used. However, low tag loss rates for PIT tags were also recorded for brown trout (4%, after seven months) and other salmonid species (0-2 %, 3 to 4 months) (Ombredane et al., 1998). Differences between the detection range of both PIT tags used (9 cm for 12 mm long vs. 30 cm for 34 mm PIT tags) produced a distinct amount of information (approximately three times more for 34 mm PIT tags) on fish behaviour confirmed by the number of repeated and non-repeated records, but higher mortality rates were found when the larger PIT tags are implanted in the smaller (< 15.0 cm) fish, in previous experiments conducted in the hatchery. However, caution should be taken

direction, although they exhibited distinct distance ranges. The dispersion of stocked brown trout is relatively well documented in the literature and, similarly to the present study, distinct individual movements were also found including stationary (maintaining the position near the stocking site) and mobile (ranging from upstream to downstream migrations) behaviour in the same stocked trout population (Cresswell, 1981). For example, Aarestrup et al. (2005) found that four brown trout left the study area whereas the remaining fish (*n*= 46) were stationary. However, in our study it was observed a decline in the water flow that conditioned the potential displacement of fish during the radiotelemetry study (32 days). Different factors can be associated with the downstream dispersal and often the harsh environmental conditions like higher levels of discharge originate the referred fish mobility (Ovidio et al., 2000), which was confirmed now. However, the river regulation (e.g. rapid fluctuations in flow resulted from hydroelectric peaking operations), water temperature regimes, different variables of physical habitat (e.g. channel slope, presence of coarse substrate particles, woody debris and roots functioning like potential cover refuges) and fish condition contribute to distinct migration behaviour of stocked fish in the natural environment. On the other hand, the influence of tagging implantation on fish movement was minimized, since external attachment of radio-transmitters and a low body/transmitter weight ratio was used, according to Brown et al. (1999). In the present study, the total distance moved (mean= 203 m) by stocked trout over the partial diel cycle considered (from 06.00 to 24.00 h) averaged about two times the length of home ranges (mean 82 m). However, total distance moved and home ranges are probably underestimated, since it was not possible to conduct registrations during a part of the night (from 00.00 to 06.00 h) and comparisons with other studies must be analysed taking into account this situation. The daily home ranges (minimum of 40 m for T4 and maximum of 118 m for T3) observed for stocked brown trout in the Baceiro stream are within the range of values observed for the same species by Young (1999) in a south-eastern Wyoming river (mean of 41 m), Ovidio et al.(2002) in a Belgian stream (8-480 m; mean 48 m), Knouft & Spotila (2002) in a Pennsylvania stream (20-2000 m) and Maia (2003) in a north-western Portuguese river (Vade River, 0-300m), excluding the migratory behaviour caused by spawning activity linked to reproduction. However, D.H.R. mentioned in the literature corresponded to resident trout and, because these fishes have a consistent fidelity to their home range or territory (Bunnel et al., 1998), it would be expected that non-resident fish, like stocked trout used in this study, had a superior mobility justifying greater dispersal, distance moved and home range, either in seasonal and diel scale analyses. Probably the dry climate during 2005 and the low discharge observed linked to high water temperatures recorded during the initial post-stocking period have restricted the movement of stocked fish. Stocked trout were shown to be more active during the day periods than during the night period in the Baceiro stream and can be related to the feeding habits acquired in the fishfarms. Brown trout is a visual feeder and the foraging efficiency decreases as light intensity declines (Fraser & Metcalfe, 1997; Klemetsen, 2003), but nocturnal (Clapp et al., 1990) and crepuscular (Bunnel et al., 1998) feeding patterns were also reported. However, the tendency of stocked trout in the Baceiro stream must be confirmed in complete (24 h) diel cycles.

Complementary analyses based on PIT-telemetry showed that only 30% of stocked fish survived five weeks after their release in the stream reach, while 92% of native trout were recaptured alive for the same period, considering the confined area of the experiment. Poor stocked brown trout survival rates were also reported in several studies (Pedersen et al.,

direction, although they exhibited distinct distance ranges. The dispersion of stocked brown trout is relatively well documented in the literature and, similarly to the present study, distinct individual movements were also found including stationary (maintaining the position near the stocking site) and mobile (ranging from upstream to downstream migrations) behaviour in the same stocked trout population (Cresswell, 1981). For example, Aarestrup et al. (2005) found that four brown trout left the study area whereas the remaining fish (*n*= 46) were stationary. However, in our study it was observed a decline in the water flow that conditioned the potential displacement of fish during the radiotelemetry study (32 days). Different factors can be associated with the downstream dispersal and often the harsh environmental conditions like higher levels of discharge originate the referred fish mobility (Ovidio et al., 2000), which was confirmed now. However, the river regulation (e.g. rapid fluctuations in flow resulted from hydroelectric peaking operations), water temperature regimes, different variables of physical habitat (e.g. channel slope, presence of coarse substrate particles, woody debris and roots functioning like potential cover refuges) and fish condition contribute to distinct migration behaviour of stocked fish in the natural environment. On the other hand, the influence of tagging implantation on fish movement was minimized, since external attachment of radio-transmitters and a low body/transmitter weight ratio was used, according to Brown et al. (1999). In the present study, the total distance moved (mean= 203 m) by stocked trout over the partial diel cycle considered (from 06.00 to 24.00 h) averaged about two times the length of home ranges (mean 82 m). However, total distance moved and home ranges are probably underestimated, since it was not possible to conduct registrations during a part of the night (from 00.00 to 06.00 h) and comparisons with other studies must be analysed taking into account this situation. The daily home ranges (minimum of 40 m for T4 and maximum of 118 m for T3) observed for stocked brown trout in the Baceiro stream are within the range of values observed for the same species by Young (1999) in a south-eastern Wyoming river (mean of 41 m), Ovidio et al.(2002) in a Belgian stream (8-480 m; mean 48 m), Knouft & Spotila (2002) in a Pennsylvania stream (20-2000 m) and Maia (2003) in a north-western Portuguese river (Vade River, 0-300m), excluding the migratory behaviour caused by spawning activity linked to reproduction. However, D.H.R. mentioned in the literature corresponded to resident trout and, because these fishes have a consistent fidelity to their home range or territory (Bunnel et al., 1998), it would be expected that non-resident fish, like stocked trout used in this study, had a superior mobility justifying greater dispersal, distance moved and home range, either in seasonal and diel scale analyses. Probably the dry climate during 2005 and the low discharge observed linked to high water temperatures recorded during the initial post-stocking period have restricted the movement of stocked fish. Stocked trout were shown to be more active during the day periods than during the night period in the Baceiro stream and can be related to the feeding habits acquired in the fishfarms. Brown trout is a visual feeder and the foraging efficiency decreases as light intensity declines (Fraser & Metcalfe, 1997; Klemetsen, 2003), but nocturnal (Clapp et al., 1990) and crepuscular (Bunnel et al., 1998) feeding patterns were also reported. However, the tendency of stocked trout in the Baceiro stream must be confirmed in complete (24 h)

Complementary analyses based on PIT-telemetry showed that only 30% of stocked fish survived five weeks after their release in the stream reach, while 92% of native trout were recaptured alive for the same period, considering the confined area of the experiment. Poor stocked brown trout survival rates were also reported in several studies (Pedersen et al.,

diel cycles.

2003; Aarestrup et al., 2005). The significant decrease in the condition of stocked brown trout suggested a lower ability to explore the available resources. In fact, the inefficient behaviours displayed by hatchery-reared fish contributed to a lesser adaptation to wild environment when compared with native trout. For example, higher aggression levels (Deverill et al., 1999), lack of social dominance structures (Jenkins, 1971), lower efficiency at feeding on wild prey (Olla et al., 1994), higher metabolic rates (Ersbak & Haase, 1983) and reduced swimming ability are reported for stocked salmonids. On the other hand, the hatchery-reared fish are more vulnerable to angling and natural predation ((Ludwig et al., 2002; Jacobsen, 2005) and higher mortality rates are associated near large stocked fish releases (Marnell, 1985). Observations from the stream bank revealed, during the experimental periods, a reduced fright response of stocked trout to human presence, confirming their potential weakness to avoid natural predation. In fact, most of the Portuguese northeastern salmonid streams supported growing populations of otter (*Lutra lutra* L.) and the higher number of spraints (4 PIT tags were detected) observed on stream banks suggested the higher mortality detected, mainly on stocked fish.

Stocked trout movements, for larger and even for smaller PIT tagged fish, were greater than for every native trout class defined. These fishes displayed an activity pattern more intense during day-light hours, namely during the dawn period. Normally, hatchery-reared trout are more active than wild fish (McLaren, 1979) and the higher mobility pattern showed during day-light hours in this study was, probably, related to the rearing environment (feeding habits in the hatchery) and the increasing ability of fish to detect food as light intensity increases (Fraser & Metcalfe, 1997). Furthermore, these results are according to the exploratory experiments made over two weeks in the previous year and confirmed the lack of a capacity to define a territory and a non-cost-effective behaviour, also detected by other authors (Bachman, 1984). In fact, the importance of habitat variables for both populations was similar during the 5 weeks suggesting that the stocked fish did not change their strategy in terms of habitat use. Hatchery-reared fish occupied habitats away from the stream banks, mainly, without aquatic cover (*e.g.* boulders, roots) and overhanging vegetation (shading). Other studies confirmed the distinct behaviour between wild and stocked salmonids based on the referred variables (Magoulick & Wilzback, 1997). Obviously, the different habitat use and the less concealment behaviour (Bachman, 1984) of stocked trout relative to wild fish increase their visibility and the vulnerability to avian and aquatic predators. A low influence of PIT tag surgical implantation in the fish peritoneal cavity was reported on different studies, with regard to survival, growth, swimming performance and general behaviour (Riley et al. 2003). During this study, trout were recaptured five weeks after their release in the stream, and healing was completed, without signs of inflammation or necrosis in the tissues, suggesting that the behaviour of both sympatric populations was not affected by the tagging procedures. Furthermore, only two stocked fish presented signs of tag expulsion, in spite of the incisions not having been closed with sutures for both 12 and 34 mm PIT tags used. However, low tag loss rates for PIT tags were also recorded for brown trout (4%, after seven months) and other salmonid species (0-2 %, 3 to 4 months) (Ombredane et al., 1998). Differences between the detection range of both PIT tags used (9 cm for 12 mm long vs. 30 cm for 34 mm PIT tags) produced a distinct amount of information (approximately three times more for 34 mm PIT tags) on fish behaviour confirmed by the number of repeated and non-repeated records, but higher mortality rates were found when the larger PIT tags are implanted in the smaller (< 15.0 cm) fish, in previous experiments conducted in the hatchery. However, caution should be taken

Combining Radio and PIT-Telemetry to Study the Large and Fine-Scale

other fish. *Journal of Fish Biology* 48: 539-541.

*Management* 22: 425-432.

871.

483: 225-230.

127: 630-636.

*Society* 119: 1022-1034.

*Evolution* 19: 334-343.

*American Fisheries Society* 129: 1373-1379.

brown trout. *Journal of Fish Biology* 50: 445-449.

fish movements. *Journal of Fish Biology* 58: 1471-1475.

Hatchery.

**7. References** 

Movements of Stocked and Wild Brown Trout (*Salmo trutta* L.) in a Northeastern Stream, Portugal 349

Forestry Governmental Services and the hatchery personnel at Marão and Castrelos Trout

Aarestrup, K., Jepsen, N., Koed, A. & Pedersen, S. (2005). Movement and mortality of

Armstrong, J.D., Braithwaite, V.A. & Rycroft, P. (1996). A flat-bed passive integrated

Armstrong, J.D. & Herbert, N.A. (1997). Homing movements of displaced stream-dwelling

Bachman, R.A. (1984). Foraging behaviour of free-ranging wild and hatchery brown trout in

Barbin-Zydlewski, G., Haro, A., Whalen, K.G. & Mccormick, S.D. (2001). Performance of

Belanger, G. & Rodriguez, M.A. (2001). Homing behaviour of stream-dwelling brook charr following experimental displacement. *Journal of Fish Biology* 59: 987-1001. Bettinger, J.M. & Bettoli, P.W. (2002). Fate, dispersal and persistence of recently stocked and

Bridcut, E.E. & Giller, P.S. (1993). Movement and site fidelity in young brown trout *Salmo trutta* populations in a southern Irish stream. *Journal of Fish Biology* 43: 889-899. Brown, R.S., Cooke, S.J., Anderson, W.G. & Mckinley, R.S. (1999). Evidence to challenge the

Bubb, D.H., Lucas, M.C., Thom, T.J. & Rycroft, P. (2002). The potential use of PIT telemetry

Bunnel, D.B. Jr, Isely, J.J., Burrell, K.H. & Van Lear, D.H. (1998). Diel movement of brown

Burrell, K.H., Isely, J.J., Bunnell Jr, D.B., Van Lear, D.H. & Dolloff, C.A. (2000). Seasonal

Castro-Santos, T., Haro, A. & Walk, S. (1996). A passive integrated transponder (PIT) tag

Clapp, D.F., Clark, R.D. Jr & Diana, J.S. (1990). Range activity, and habitat of large, free-

Clarke, K.R. & Warwick, R.M. (1994). *Change in marine communities: An approach to statistical* 

Cortes R.M.V., Teixeira A. & Pereira C. (1996). Is supplemental stocking of brown trout (*Salmo trutta*) worthwhile in low productive streams? *Folia Zoologica* 45: 371-381

Clarke, K.R. & Gorley, R.N. (2001). *Primer v5: User Manual/Tutorial*. PRIMER-E Plymouth. Cooke, S.J., Hinch, S.G., Wikelski, M., Andrews, R.D., Kuchel, L.J., Wolcott, T.G. & Butler,

system for monitoring fishways. *Fisheries Research* 28: 253-261.

transponder antenna array for monitoring behaviour of Atlantic salmon parr and

stationary and portable passive transponder detection systems for monitoring of

resident rainbow trout in a Tennessee tailwater. *North American Journal of Fisheries* 

"2% rule" for biotelemetry. *North American Journal of Fisheries Management* 19: 867-

for identifying and tracking crayfish in their natural environment. *Hydrobiologia*

trout in a southern Appalachian river. *Transactions of the American Fisheries Society*

movement of brown trout in a Southern Appalachian River. *Transactions of the* 

ranging brown trout in a Michigan stream. *Transactions of the American Fisheries* 

*analysis and interpretation.* Natural Environment Research Council. London. 144 pp.

P.J. (2004). Biotelemetry: a mechanistic approach to ecology. *Trends in Ecology and* 

stocked brown trout in a stream. *Journal of Fish Biology* 66: 721-728.

a stream. *Transactions of the American Fisheries Society* 113: 1-32.

in the interpretation of data, since a low proportion of area (eight panel antennae) was sampled for every diel cycle. This limitation was reported in several studies using PIT telemetry technology and further improvements are needed to increase the detection range of PIT reading units. With regard to the experimental design of this study, protocols combining a superior number of stationary flat-bed antennas and MPD units covering, at the same time period, the entire stream reach selected, and the use of portable antenna technology will improve the quality of data acquisition related to the small-scale movements by sympatric stocked and native trout populations.
