© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Observations of natural behaviour of yellowtail flounder derived from data storage tags
Northwest Atlantic Fisheries Centre, Department of Fisheries and Oceans, Science, Oceans and Environment Branch PO Box 5667, St. John's, Newfoundland, Canada, A1C 5X1
*Correspondence to S. Walsh. e-mail: walshs{at}dfo-mpo.gc.ca.
Electronic data storage tags (DSTs) were used to monitor natural behaviour of adult yellowtail flounder (Limanda ferruginea) in relation to depth, temperature, time of day, and season. Over a 2-year period, a total of 543 fish tagged with DSTs were released during June of each year. Results from 29 tags revealed that yellowtail flounder exhibit diel and seasonal variations in depth and temperature. The results also show, for the first time in this species, that during various times of the year yellowtail flounder make extensive off-bottom movements at night and can remain off bottom for several hours.
Keywords: behaviour, data storage tags, diel, flatfish, movements, surveys, yellowtail flounder
Received 11 July 2003; accepted 5 May 2004.
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Introduction |
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Yellowtail flounder (Limanda ferruginea) reaches its northern limit of commercial concentration on the Grand Bank and constitutes an important bottom-trawl fishery for the Canadian fleet (Walsh, 1992). The largest proportion of the stock is found mainly in the eastern area of the southern Grand Bank in depths of 40–70 m and rarely beyond 100 m (Walsh, 1992). This small-mouth flounder is expected to show typical flatfish behaviour by spending prolonged periods on or close to the seabed. It feeds primarily on surficial and interstitial benthic macrofauna, mainly polychaetes and amphipods (Pitt, 1976). Spawning is thought to be widespread throughout the southern area of the bank during the summer months, with a peak in late June–early July (Pitt, 1970). Neither feeding nor spawning migrations or aggregations have been identified.
Outside temporal and spatial analyses of catch data from annual surveys and the commercial fishery little is known about the daily behaviour of yellowtail flounder. Observations of movements from traditional tagging studies have suggested that Grand Bank yellowtail flounder are relatively sedentary and make moderate movements from their release sites (Walsh, 1987; Morgan and Walsh, 1999; Walsh et al., 2001). Although tagging yellowtail flounder using traditional techniques, such as Petersen discs, can provide information on individual movements, i.e. release and recapture positions, it may underestimate the true extent of their movements. Recent electronic data storage tagging (DSTs) studies of North Sea plaice (Pleuronectes platessa) used measurements of depth and temperature to reconstruct the tracks of plaice movements (Metcalfe et al., 1994). It was discovered that the rates of movements were often 10 times faster and farther than those deduced from traditional mark–recapture experiments.
Knowledge of yellowtail flounder movements and behaviour is important to the assessment and management of this fishery resource, helping to explain anomalies in abundance indices from surveys and changes in fishing patterns of the commercial fleet. In this study, we use DSTs to study the natural behaviour of yellowtail flounder on the Grand Bank. Preliminary investigations focused on diel and seasonal changes in activity patterns with respect to depth and temperature.
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Material and methods |
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Study area
The Grand Bank is a raised submarine plateau situated southeast of Newfoundland, measuring approximately 93 200 km2. On this plateau, water depths range from 36.5 m to 185 m, with most of the area being less than 100 m. The bank is dominated by the southward flowing cold, low saline Labrador Current, flowing over the bank and the warmer Gulf Stream flowing northwest along the eastern edge and sometimes sweeping across the southern area of the bank (Helbig et al., 1992).
Experimental design
There were two releases of electronic data storage tags: one release of 310 tags in June 2001 and other release of 233 tags in June 2002. The taggings occurred 1 or 2 weeks before the estimated peak spawning period of late June–early July and corresponded to the summer spawning closure (June 15–July 31) of the fishery to the Canadian fleet. Fish were captured, tagged, and released in near equal numbers at 25 stations on the Grand Bank in 2001 and at 31 stations in 2002 (see Walsh et al., 2001 for details). Fish were caught by a commercial trawling vessel using a tow duration of 10–15 min and a towing speed of 1.5 ms–1. Only fish greater than 40 cm (range 40–54 cm) and in good condition were tagged with DSTs. All fish of this size are sexually mature (Walsh and Morgan, 1999), but their sex was not determined. After keeping fish in holding tanks for at least 15 min, the tags were attached to the dorsal pigmented side anterior to the widest section of the fish using two attachment points on either end of the tag (see Walsh et al., 2001 for details). In the first year, we used 187 DST 200 tags, each weighing 1 g in water and 123 DST 300 tags, each weighing 1.5 g in water (Star Oddi, Iceland). In the second year, we used 233 DST Milli tags, each weighing 5 g in water (Star Oddi, Iceland). Each of the 543 tags was programmed to record both depth (0–800 m) and temperature (–2 to 20°C) every 32 min to ensure collection of data for a minimum 9 months depending on storage capacity of the tag. The resolution on the depth measure for DST 200 and 300 tags is 3 m and for DST Milli tags it is 0.23 m (Star Oddi, Iceland).
Analyses
Twenty-nine DSTs (18 DST 200–300 and 11 DST Milli) (5% of the releases) returned from the commercial fishery to date (March 2003) were used in this analysis (Table 1). The release and capture information were used to remove the initial tag recordings at the surface, the time it took to reach bottom and recordings after capture. The majority of fish reached bottom within the first 32 min after release. The absolute accuracy of depth and temperature recordings differs between tags. However, the data have not been adjusted to correct for absolute accuracy prior to any analysis, and this should have little impact on the interpretation of results.
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Activity levels
Simple parametric statistics were used to look at changes in depth and temperature over a 24-h period and also by month to investigate seasonal changes. Each day was divided into three periods; night, day, and crepuscular, and records from each tag were assigned to one of the three periods using the onboard clock data. Sunrise and sunset were calculated based on the method of Brock (1981) and standardized using the latitude (45°N), a position central to the release sites. The crepuscular period was defined as one-half hour before to one-half hour after sunrise and sunset. The range of depth or temperature was calculated for each fish for each period of the day on each day the fish was at liberty.
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Results |
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Dispersion patterns
Returned fish ranged in size from 41 to 54 cm (Table 1). The number of days at liberty ranged from 87 to 514 days [181.6 ± 23.0 days (mean ± s.e.)] with 25 out of 29 tags being at liberty for less than 200 days and four tags being at liberty for more than 400 days.
The direction and distance of return position relative to release position were calculated for each tag. The average distance travelled was 69.2 ± 9.8 km (mean ± s.e.) with the maximum distance being 225.9 km. Tags were returned from every compass direction except north, being more frequently returned from positions south (76%) of the release areas (Table 1).
Seasonal changes in depth and temperature
A seasonal change in mean depth and temperature occupied by yellowtail flounder was evident (Figure 1). There were only minor differences in means of the three daily time periods with mean depth at night being slightly lower than that measured during the day and crepuscular periods from April to September. Average depths ranged from 60 m to 75 m, with a trend for yellowtail flounder to occupy shallower depths from June until December, and deeper and more variable depths from January to May. The average temperatures occupied by yellowtail flounder in the winter showed a decrease from 2.8°C in January to 0.5°C in March, followed by an increasing trend through the spring, summer, and early fall to reach a maximum of 3.9°C in October before decreasing to an average of 2.7°C by December. This increasing trend in temperature, in particular from May to October, corresponded to the decreasing trend in average depths occupied.
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Vertical movement patterns
All 29 tags showed vertical movements off bottom, and these movements varied throughout the year, being more prevalent during July–October than at other times of the year. However, this summer–fall pattern could be misleading, since 25 of 29 tags covered only the period June–November, while the remaining four tags were from yellowtail flounder at liberty for more than a year. It was evident that the mean range in depths occupied by yellowtail flounder was larger during night-time compared to daytime and crepuscular periods, especially from June to September (Figure 2).
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The plot of each tag's measurement data for depth and temperature during its period at liberty showed substantial vertical activity (see Figure 3 for examples). There were three basic patterns in the depth data: (i) periods of very limited vertical movements, (ii) periods of relatively frequent vertical movements, and (iii) periods of no detectable vertical movements. Most of the vertical movements occurred at night. Night-time movements often lasted for several hours with occasional descents back to the bottom (see Figure 4 for an example). Coincidental with some of these vertical movements was a change in recorded temperatures (Figures 3, 4). It was evident that yellowtail flounder were crossing the thermocline during the summer.
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Discussion |
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This study shows evidence of seasonal shifts in average depths and temperature occupied, and also diel differences in the range of depths occupied by yellowtail flounder. Based on the DST data, yellowtail flounder appear to occupy a relatively narrow range in temperatures (0.4–4.3°C) and depths (56–83 m) on the Grand Bank. However, the temperature (at least up to 12°C) and depth ranges are more extensive when yellowtail undertake vertical movements into the midwater column. Earlier studies on the Grand Bank (Walsh, 1992) and on the Scotian Shelf (Perry and Smith, 1994) have shown that yellowtail flounder can tolerate a wide range of temperatures, but stay within a narrow depth range. It is possible that the prominent trend in temperatures recorded by the DSTs is reflective of the normal seasonal changes in the warming and cooling of the bottom layers on the Grand Bank. However, the analyses presented here are not sufficient to determine the effect of temperature selection on the movements of yellowtail flounder. The results of this study also indicated that the mean range in depths occupied by yellowtail flounder is significantly higher at night than during the day, especially during the summer months. We attribute these highly variable ranges to yellowtail flounder undertaking vertical movements in the water column, similar to North Sea plaice, and not to yellowtail moving up onto shallow bottom areas at night and back to deeper water during the day.
Evidence of vertical movements in yellowtail flounder was detected in all tags returned. From the depth and temperature records of the two yellowtail examined in detail here, vertical movements were higher at night, often lasting for several hours and generally occurred more frequently during the summer and early fall periods than at other periods of the year. However, these seasonal observations may be biased, given that only four returned tags cover the period from December to May. Extensive vertical movements with a diel and/or a tidal periodicity have been documented in North Sea plaice and were linked to either off-bottom movements to different areas on the feeding grounds or to migration using tidal stream transport during both day and night (see Arnold and Holford, 1995 for review). Short-term changes in vertical movements are also reported to be related to foraging behaviour of cod (Gadus morhua) (Godø and Michalsen, 2000), predator avoidance in walleye Pollack (Theragra chalcogramma) (Sogard and Olla, 1997), and spawning behaviour in Icelandic plaice (Pleuronectes platessa) (Solmundsson et al., 2003). Changes in the range, extent, and periodicity of these vertical movements seem to define whether the activity is linked to one of these specific behaviours.
It is evident that these extensive vertical migrations shown here can bring yellowtail flounder into warmer waters, even crossing the thermocline in the summer, and this may be a way of behaviourally supplementing the metabolic potential necessary to carry out activities such as locomotion, spawning, foraging, or escaping (Claireaux et al., 2000). Yellowtail flounder are daytime bottom feeders, while most discernible vertical movements occur during the night. Since yellowtail's peak spawning is in late June and early July, it is unlikely that these movements are solely related to spawning, since most vertical movements were prevalent from July to October. The large size of yellowtail (>40 cm) would preclude the possibility that they are avoiding predators by moving off bottom. Whether these vertical movements are carried out by yellowtail to take advantage of tides to move to other areas is unknown at present. The mainly southward directional movements from our DST release sites on the bank during the first 6 months after release (post-spawning) may reflect this, or they could also reflect the restricted geographical extent of the yellowtail flounder fishery. No attempt, in this preliminary analysis, has been made to analyse the DST data for tidal changes because 19 of the 29 tags returned have a depth resolution (3 m) that makes them incapable of measuring the rise and fall of the tides. In addition, there is little or no published information about tides on the Grand Bank in the areas where the tags were released and recovered.
To understand the potential cause of vertical movements and subsequent variation in availability of yellowtail flounder to survey and commercial gears, more knowledge of individual fish behaviour is needed, such as the extent of daily vertical range, number, frequency and speed of ascents and descents, resident time in midwater, and repeatability of vertical movements. In addition, data on tides are needed to examine their role in the off-bottom movements of yellowtail flounder. A tidal/current gauge mooring will be deployed in the fall of 2003 to gather these data.
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Acknowledgements |
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We thank NAFC personnel, L. Mansfield for coordinating tagging studies, P. Upward for data extraction and cataloguing, and W. B. Brodie for project support. Special thanks go to the fishing crew of the FPI "Atlantic Claire". Thanks to Star-Oddi for being helpful in many technical aspects. This project was jointly funded by the Department of Fisheries and Oceans Canada and Fishery Products International Inc. of St. John's, Newfoundland.
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References |
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