the effect of weather conditions on harbour seal ( phoca vitulina )...

The effect of weather conditions on harbour seal (Phoca vitulina) haulout behaviour in the Moray Firth, northeast Scotland

Kate Grellier, Paul M. Thompson, and Heather M. Corpe

Abstract: Abundance data from a 6-year study (1988- 1993) of harbour seals (Phoca vitulina) were used to control for seasonal changes in haulout behaviour and assess the effect of temperature and other weather conditions on haulout numbers at a site in Scotland. A significant relationship between Julian day and haulout numbers was consistent between years. In some years, there was also a significant relationship between ambient temperature and haulout numbers, but examination of the residuals around the relationship between haulout numbers and Julian day revealed no evidence for a consistent effect of temperature, wind speed, or wind-chill adjusted temperatures. A weak negative relationship between haulout numbers and, both precipitation and cloud cover did exist, but these variables explained little of the variation in seal numbers.

RCsumC : Des donnkes sur l'abondance des Phoques communs (Phoca vitulina) au cours d'une ktude de 6 ans (1988- 1993) ont servi a itudier l'impact des changements saisonniers sur les skjours a terre et a kvaluer l'effet de la tempkrature et des autres conditions climatiques sur le nombre de phoques a terre a une station situke en ~ c o s s e . Nous avons constatk l'existence d'une relation significative constante d'annke en annke entre la date et le nombre de phoques a terre. Certaines annkes, il y avait kgalement une relation significative entre la tempkrature ambiante et le nombre de phoques a terre, mais l'examen des valeurs rksiduelles qui ne font pas partie de la relation entre le nombre d'individus a terre et la date n'a pas mis en lumikre d'effets constants de la tempkrature, de la vitesse du vent ou des tempkratures ajustkes en fonction du facteur vent. Une faible corrklation negative entre le nombre d'individus a terre et la combinaison prkcipitations-couverture de nuages a kt6 enregistrke, mais ces variables n'expliquent qu'une petite partie de la variation du nombre de phoques a terre. [Traduit par la Rkdaction]

Introduction

Pinnipeds forage at sea, but remain dependent on land or ice for at least part of their annual cycle. In all species, adult females must return ashore to give birth, but there is both inter- and intra-specific variation in the extent to which seals remain ashore at other times (Reidman 1990). In the past, most interest in these haulout patterns has been related to the use of terrestrial counts to estimate pinniped abundance and distribution (Eberhardt et al. 1979; Thompson and Harwood 1990). More recently, interest in disease as a factor influenc- ing pinniped population dynamics has highlighted the impor- tance of terrestrial haulout for transfer of pathogens within and between populations (Harwood and Hall 1990; Heide- JBrgenson et al. 1992; Thompson and Hall 1993).

For many species living in polar and subpolar regions, weather conditions can have a marked effect on haulout behaviour. For example, Weddell seal (Leptonychotes wed- delli) haulout numbers have been significantly correlated with wind velocity (negatively), temperature (positively),

Received October 12, 1995. Accepted March 12, 1996.

K. Grellier,' P.M. Thompson, and H.M. Corpe. University of Aberdeen, Department of Zoology, Lighthouse Field Station, George Street, Cromarty, Ross-shire, I V l l 8YJ, Scotland.

Author to whom all correspondence should be addressed (e-mail: k.grellier@abdn.ac.uk).

and solar radiation (positively) (Smith 1965; Ray and Smith 1968; Wartzok 1991). Similarly, the amount of time that harp seals (Phoca groenlandica) spent hauled out was closely correlated with hours of sunshine, and there were weaker correlations with wind speed and temperature (Stewart 1987).

Studies such as these have led to the suggestion that higher temperatures may increase the number of seals hauling out and the degree of aggregation at haulout sites, thus increas- ing the incidence or severity of disease (Lavigne and Schmitz 1990). In particular, Lavigne and Schmitz (1990) highlighted the fact that the phocine distemper virus (PDV) epizootic, which affected European harbour seal (Phoca vitulina) popu- lations in 1988, coincided with a series of unusually mild summers. In turn, they suggest that global warming may fur- ther affect marine mammal populations through similar out- breaks.

Previous studies have shown that extreme temperatures, or a combination of wind and temperature, can have a marked negative effect on harbour seal behaviour (Boulva and McLaren 1979; Watts 1992; Hansen et al. 1995). The extent to which weather conditions influence the haulout behaviour of harbour seals in more temperate regions, including those European and North Sea waters where the PDV outbreak was most severe (Heide-JBrgenson et al. 1992), is less clear. There are no published studies from European waters; however, correlations between haulout numbers and weather conditions that have been found in other regions are generally weak and, more importantly, are

Can. J. Zool. 74: 1806- 181 1 (1996). Printed in Canada 1 ImprimC au Canada

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often inconsistent across studies. For example, Godsell (1988) found no significant correlation between number of harbour seals hauled out and wind speed, but a significant correlation with precipitation. In contrast, Kovacs et al. (1990) found a significant correlation between numbers and wind speed, but no significant correlation with precipitation.

In a reply to Lavigne and Schmitz's (1990) paper, Harwood and Grenfell (1990) argue that our understanding of the influence of weather conditions on harbour seal behaviour is too limited to allow conclusions to be drawn concerning the role of changing climate on previous or future disease outbreaks. In particular, they highlighted the fact that harbour seals breed and moult during the warmer summer months, and that these activities may themselves lead to an increase in the time that seals spend ashore. Thus, studies must be conducted over several years to control for season and evaluate the relative influence of breeding and moult cycles and ambient temperature on harbour seal haulout behaviour. However, previously published studies of this type have generally been carried out over only 1 (e.g., Kovacs et al. 1990; Godsell 1988) or 2 (Kreiber and Barrette 1984; Pauli and Terhune 1987; Watts 1992) years. Conse- quently, some of the relationships reported may simply result from correlations between season and certain meteorological variables.

The objective of this study was to explore the effects of temperature on harbour seal haulout behaviour in a temper- ate area, northeast Scotland. In particular, we used a time- series extending over 6 years to control for time of year and to determine the influence of weather conditions on the number of seals hauling out at this site.

The study was carried out at an intertidal haulout site in the Cromarty Firth, northeast Scotland (57"37.4'N, 4'21 .OtW). The sandbanks lie in relatively sheltered waters but are most exposed to winds from the southwest and northeast. Sandbanks are uncovered for 2 - 3 h either side of low tide and are used by both reproductive and nonreproductive harbour seals throughout the year. Harbour seals were counted, using either a 15 -45 x 60 or 20-60 x 80 telescope, between 5 and 36 times a month during June and July (Julian days 152 -2 13) of 1988 - 1993. Each count was made within 3 h of low tide. Up to 46 pups were counted at this site during the peak of the pupping season (late June - early July), but all pups were excluded from further analysis. Where more than one count was made on a single low-tide cycle, the highest value was used. This value is subsequently referred to as the low-tide count. Meteorological data were recorded routinely every 3 h by the Cromarty Firth Port Authority at Invergordon Harbour (57'41 .OtN, 4"10.01W), approximately 13 km northeast of the study site. The following data were available for analysis: air temperature ("C); wind speed (knots); wind direction (0-360"); pressure (millibars; 1 mbar = 100 Pa); cloud cover (eighths of total sky, or octas), and precipitation (presence or absence). These data were also used to calculate the air temperature adjusted for wind chill (subsequently referred to as adjusted temperature):

where T, is adjusted temperature ("C), To is observed temperature ("C), and W is wind velocity (km . h-I) (Boulva and McLaren 1979). Data on the predicted time and height of each low tide were taken from the local nautical almanac (Willis 1988- 1993).

Initially, one- and two-degree polynomial regression analyses

Table 1. Mean, standard error, minimum, and maximum values for temperature ("C), adjusted temperature ("C), wind speed (knots) and direction ("), pressure (millibars), and cloud cover (octas) for each year of the study.

Year Mean SE Min. Max.

Temperature

Adjusted temperature

Wind speed

Wind direction

Pressure

Cloud cover

(Unistat Ltd., London) were used to determine whether there were within-year relationships between low-tide counts of seals and different meteorological variables. Because there were between- year differences in the annual maximum count obtained during the study period, each count was converted to a percentage of that

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Fig. 1. Relationship (two-degree polynomial regression) between percentage count and Julian day. Data for all 6 years are pooled; r2 = 0.27, p < 0.01, n = 228; y = -0.032r2 + 11.940~ - 1017.665.

,Ju~an Day

Table 2. Two-degree polynomial regression results for Julian day and adult harbour seal haulout numbers throughout the season studied (June and July) for each year (in the study period) separately; maximum counts for each year (and dates on which they were made) are also shown.

- -

Max. Year n r2 P count Date Regression equation

1988 10 0.331 0.245 109 4 July y = -0.010x2 + 4 . 3 2 8 ~ - 386.182 1989 50 0.293 <0.01 93 14 July y = -0.012r2 + 4 . 9 0 5 ~ - 403.146 1990 36 0.394 <0.01 87 16 July y = -0.022r2 + 8 . 5 7 4 ~ - 742.075 1991 20 0.234 0.104 102 17 July y = -0.021x2 + 7 . 9 3 3 ~ - 662.556 1992 64 0.447 <0.01 100 29 June y = -0.037x2 + 13 .272~ - 1120.374 1993 48 0.395 <0.01 106 1 1 July y = -0.054x2 + 19 .280~ - 1646.416

Note: Significant results are shown in boldface type.

year's maximum count to permit comparison between years (Watts 1992). These adjusted counts are subsequently referred to as per- centage counts.

The dominant relationship in all years where sample sizes exceeded 30 was between low-tide count and Julian day (see the Results). To assess whether weather conditions further explained any variation in low-tide counts, percentage counts for all years were pooled and regressed against Julian day. The residuals of this relationship (subsequently referred to as residual counts) were first related to different meteorological variables using both one- and two-degree polynomial regression analyses. Secondly, a Mann - Whitney U test was used to compare the medians of the negative and positive residual counts for each environmental variable.

Because precipitation was measured only as presence or absence, the relationship between the residual count and rainfall could not be measured using regression analyses. Instead, residual counts in the

presence and absence of precipitation were compared using a Mann - Whitney U test.

Results

During the study period, temperatures varied from 8.7 to 23.3 OC and adjusted temperature varied from 1.3 to 26.2OC, a range of 14.6 and 24.9"C, respectively (Table 1). For all years in which >30 counts were made, there was a signifi- cant two-degree polynomial relationship between low-tide count and Julian day (Table 2). Numbers rose to a peak in midseason (approximately Julian day 183) and then decreased. When percentage counts from all 6 years were pooled, there remained a highly significant relationship between percen- tage count and Julian day (Fig. 1). There were occasional

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Table 3. Two-degree polynomial regression results for different environmental variables and adult harbour seal haulout numbers for each year (in the study period)

Temperature r2 P

Wind speed r2

P Wind direction r2

P Cloud cover r2

P Adjusted temperature r2

P

Note: Significant results are shown in boldface type.

Table 4. Probability values for one- and two-degree polynomial regression analyses (variables against residual counts).

Table 5. Probability values for Mann- Whitney U tests carried out on positive and negative residual counts for each environmental variables.

Variable One-degree Two-degree Variable P

Time of day 0.019 0.000 Temperature 0.085 0.178 Wind speed 0.823 0.433 Wind direction 0.962 0.384 Cloud cover 0.546 0.010 Adjusted temperature 0.125 0.307

Note: Significant results are in boldface type.

significant relationships between low-tide count and other variables (e.g . , temperature in 1989), but none of these rela- tionships were consistent between years (Table 3).

Examination of the distribution of residual counts from the relationship between percentage count and Julian day (Fig. 1) showed that time of day was the other key factor influencing haulout counts (Table 4), although r2 values were only 0.02 and 0.07 for one- and two-degree polynomial regressions, respectively. Residual counts were not related ( p > 0.05) to either temperature, wind speed, wind direc- tion, or adjusted temperature (Table 4). There was a signifi- cant correlation with cloud cover (Table 4, Fig. 2), although the r2 value was only 0.04. There was no significant differ- ence between the values of the positive and negative residual counts for any of the environmental variables (Table 5).

Precipitation was shown to have a significant effect on haulout numbers. Residual counts in the presence of precipi- tation were significantly lower than those in the absence of precipitation (Mann-Whitney U test, p = 0.037, z = -2.085). However, the effect of precipitation was slight in that median number of harbour seals hauled out was 75.5 on days when there was no precipitation (202 days) compared with 69.5 on days when there was precipitation (26 days).

Discussion

Although there were occasional significant relationships between low-tide counts and environmental conditions (Table 3), the only consistent trend between years was with

Temperature Wind speed Wind direction Cloud cover Adjusted temperature

Julian day (Table 2, Fig. 1). This midsummer peak in numbers coincides with the annual pupping season, when females have been shown to spend more time ashore (Thompson et al. 1989, 1994). If environmental conditions were having an important effect on seal haulout behaviour, there should be a consistent relationship between one or more of these factors and the residuals from this dominant seasonal trend. However, the only significant relationships found were between the residual counts and precipitation and cloud cover. These explained little of the observed variation and were presumably related variables.

We therefore found no evidence to support the suggestion that variations in temperature, with or without appropriate adjustments for wind chill, influenced the haulout patterns of harbour seals in this area. While these conclusions differ from those from some studies (Boulva and McLaren 1979; Godsell 1988; Kovacs et al. 1990), several important differ- ences should be noted. Firstly, other studies have generally been short term. As suggested by Harwood and Grenfell (1990), apparent temperature effects may result from inter- actions with seasonal trends and we suggest that this may explain the significant relationships found in 1989 and 1990 (Table 3). This highlights the need for a longer time-series of data in order to investigate the influence of temperature. Secondly, the climate in our study area was temperate, whereas other studies have often been conducted under more extreme conditions (Boulva and McLaren 1979). Finally, our study was conducted in summer and it is possible that the behaviour of seals in this area was more strongly affected by weather in winter. Nevertheless, interest in the role of

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Fig. 2. Relationship between residual count and temperature ( a ) , adjusted temperature (b) , wind speed (c) , and cloud cover (d) (when all 6 years are pooled).

(a) (b)

40 1 . . 40 I .

Temperature ("C) Adjusted Temperature ("C)

Wind Speed (kn)

weather on haulout behaviour has principally been in relation to its role in the design of population surveys (Eberhardt et al. 1979; Olesiuk et al. 1990) and its influence on disease transfer rates (Lavigne and Schmitz 1990). It therefore seems appropriate for studies to concentrate on this period because annual abundance tends to peak during the summer breeding season and moult.

The lack of any relationship between temperature and haulout patterns is at variance with the expectation that harbour seals may obtain thermoregulatory benefits from coming ashore during warmer conditions. However, studies of foraging patterns suggest why this may be so. In summer, Moray Firth harbour seals typically feed 20-80 km from haulout sites and remain at sea for periods of between 1 and 12 days (Thompson and Miller 1990; Thompson et al. 1994; unpublished data). Between foraging trips seals return inshore for periods of several days and generally haul out on each low tide. The relatively long travel time between forag- ing and haulout areas means that there is unlikely to be a close relationship between the timing of individual haulout boyts and day-to-day variations in weather conditions. There- fore, where seals use distant foraging areas, haulout patterns are likely to be most strongly influenced by feeding require- ments or foraging success, although haulout numbers may be depressed by extreme conditions. Feeding trips lasting several days have been recorded in a number of studies

-60 T I I I I I 1 I

0 1 2 3 4 5 6 7 8

Cloud Cover (octas)

(Thompson et al. 1989; Allen 1988), but it remains possible that some populations may forage closer to haulout sites. In such areas there may be a stronger relationship between weather conditions and haulout patterns. In our study area, however, it appears that foraging activity and other seasonal events within the seals' annual cycle are a more important influence on summer haulout behaviour than temperature.

Acknowledgements

We thank Karen Gardiner, David Miller, and David Wood for help during fieldwork, and the Cromarty Firth Port Authority and the Meteorological Office for providing weather records. John Harwood, John Hislop, Vincent Janik, Ann Mackay, Dominic Tollit, Shelagh Parlane, and two anonymous reviewers provided valuable comments on earlier drafts of the manuscript. This work was carried out under a series of contracts to Prof. P. A. Racey and Dr. P.M. Thompson from the Scottish Office Agriculture, Environ- ment and Fisheries Department.

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