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Relationships between reproductive performance and organochlorine contaminants in great black-backed gulls (Larus marinus)
Environmental Pollution 134 (2005) 475–483 www.elsevier.com/locate/envpol
Relationships between reproductive performance and organochlorine contaminants in great black-backed gulls (Larus marinus) Morten Helberga,b, Jan Ove Bustnesb,*, Kjell Einar Erikstada,b, Kai Ove Kristiansena, Janneche Utne Skaarec a Faculty of Science, Department of Biology, University of Tromsø, 9037 Tromsø, Norway Norwegian Institute for Nature Research, Unit for Arctic Ecology, The Polar Environmental Centre, N-9296 Tromsø, Norway c National Veterinary Institute, P.O. Box 8156 Dep., N-0033 Oslo, Norway
b
Received 2 April 2004; accepted 8 September 2004
Elevated blood concentrations of organochlorine contaminants correlate with poor reproductive performance in female great black-backed gulls. Abstract The great black-backed gull Larus marinus is a top predator in subarctic and temperate marine ecosystems, and the aim of this study was to investigate if organochlorines (OCs) were related to reproductive performance in this species at the subarctic parts of the Norwegian Coast. We measured blood levels of various OCs in 53 breeding birds. The OC levels were relatively low compared to levels found in nearby arctic areas. In females, however, there was a significant positive relationship between blood concentrations of OCs, especially hexachlorobenzene (HCB) and, p,p#-dichlorodiphenyldichloroethylene (DDE), and egg laying date, and a positive relationship between the probability of nest predation and blood concentration of b-hexachlorocyclohexane (b-HCH), oxychlordane, and DDE. In females with high levels of OCs, especially persistent polychlorinated biphenyls (PCBs), there was also a decline in egg volume as egg laying progressed; i.e. the second and third laid egg were relatively smaller, compared to females with low OC levels. No relationships between reproductive parameters and OC levels were found in males. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Subarctic; POPs; Reproductive output; Egg size; Nest predation; Egg laying date
1. Introduction Factors that cause environmental change beyond the range that wildlife normally are adapted to are likely to affect population growth and maintenance. One such factor is anthropogenic pollutants such as
* Corresponding author. Tel.: C47 77 75 04 07; fax: C47 77 75 04 01. E-mail address: [email protected] (J.O. Bustnes). 0269-7491/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2004.09.006
organochlorines (OCs), e.g. PCBs and DDT, which are distributed in food webs globally, and are known to have a wide array of biochemical, physiological and ecological effects (Jones and de Voogt, 1999; Burger et al., 2001). Food is the major source of OCs and due to their lipophilicity and persistent nature they biomagnify to their highest levels and cause greatest effects in top predators (Jones and de Voogt, 1999). For example, in the marine food web seabirds such as gulls Larus sp. are top predators, and probably particularly sensitive to OCs because they accumulate lipid reserves during parts
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of the year (Walker, 1990). For more than three decades, gulls such as the herring gull Larus argentatus have been studied in relation to accumulation and effects of OCs, particularly in the highly industrialized Great Lake District in North America (e.g. Herbert et al., 1999). However, high concentrations of OCs are not only found in industrialized areas, and even in the remote Arctic long-transported OCs, especially PCBs and DDE, have been found in alarmingly high levels, especially in top predators such as polar bears Ursus maritimus and glaucous gulls Larus hyperboreus (e.g. Bogan and Bourne, 1972; Gabrielsen et al., 1995; Bernhoft et al., 1997). Recently, a series of studies of the glaucous gull at Bear Island (74 30# N, 19 01# E) in the Norwegian Arctic (Fig. 1) have found this population to be negatively affected by OCs, including impaired reproduction and lowered survival (Sagerup et al., 2000; Bustnes et al., 2001a, 2002, 2003, 2004). However, virtually nothing is known about potential effects of OCs in seabirds along the Norwegian Coast which stretches from the industrialized North Sea area up to subarctic areas (Fig. 1). Previous seabird studies in this area have only determined levels and composition of OCs from seabird eggs and individuals (e.g. Fimreite et al., 1980; Moksnes and Norheim, 1986; Barrett et al., 1996; Henriksen et al., 1996). The aim of this study was
N
Bear Island
500 km
Loppa
#
to document whether OCs were having similar effects on the subarctic parts of the Norwegian Coast as at Bear Island about 500 km further north. We studied the great black-backed gull Larus marinus since it has a similar ecology as the glaucous gull, i.e. it has an opportunistic feeding ecology and it can also be a fierce predator (Cramp and Simmons, 1983; Good, 1998). High levels of OCs have also been found in this species in some areas (Weseloh et al., 2002). In arctic and subarctic areas, most of the environmental pollutants are long-transported and originate from non-point sources. A common feature of longtransported contaminants is that the concentrations of the different compounds are highly correlated. Accordingly, individuals with high levels of, for example, PCBs have relatively high levels of all persistent congeners and of other persistent OCs, such as hexachlorobenzene (HCB), b-hexachlorocyclohexane (b-HCH), oxychlordane, and p,p#-dichlorodiphenyldichloroethylene (DDE) (Mora et al., 1993; Henriksen et al., 1998; Jones and de Voogt, 1999; Bustnes et al., 2001b). In field studies it is thus difficult to document specific effects of different contaminants or contaminant groups (Bustnes et al., 2003). It is possible that single compounds are responsible for the observed effects, but additive effects of subgroups of the contaminants, or synergistic and antagonistic effects, may also be important. However, the degree of correlation between different compounds varies; i.e. some individuals are having relatively high concentrations of some compounds but not others, and it may be possible to exploit this variation and test if some compounds are more often stronger predictors of negative effects than others (Bustnes et al., 2003). In this study we used this approach, and measured the concentrations of pesticides (HCB and b-HCH) or pesticide metabolites (DDE and oxychlordane) and eight PCB congeners (six persistent congeners) in the blood of great black-backed gulls and correlated these levels to various reproductive parameters, including egg laying date, nest predation, egg size and clutch size.
2. Materials and methods
No
rw
ay
2.1. Study area
North Sea
Fig. 1. Map showing the study colony, Loppa Island, in relation to the Norwegian Arctic.
The study was carried out at Loppa Island (70 20# N, 21 24# E), Finnmark County, in northern Norway (Fig. 1). The island is 7.4 km long and 2.5 km wide and the highest peak (Rektind) is 274 m above sea level. This island provides a wide variety of habitats for breeding birds, varying from sheer cliffs at the western side, rocky shores at the northern and eastern side, and a sand-dune area in the southern part. The island has a plateau level that ranges from 100 m above sea level in the southern part, and rises up to 200 m in central parts and gradually
M. Helberg et al. / Environmental Pollution 134 (2005) 475–483
decreases down to 50 m above sea level in the northern end. In the western steep cliffs there are colonies of Atlantic puffins Fratercula arctica, razorbills Alca torda, and common murresUria aalge. The great black-backed gull breeds mostly in solitarily pairs or in loose colonies, sometimes with other gull species. In 2001 there were about 150 pairs breeding at Loppa, and the preferred habitat was at the plateau or around the rocky shores, and only a few pairs were breeding in the forest and lowland areas. 2.2. Methods From 27th of April and onwards all nests were checked daily if possible. The first eggs were found on 29th, and all eggs were marked with a marker pen and the nests were marked with a unique number. The axial length and breadth of each egg in the laying sequence (first, second and third eggs: designated egg a, b, and c) was measured with a digital caliper to the nearest 0.05 mm. Repeatability of egg volume was very high (df Z 267, R2 Z 0.9987, P ! 0.0001). The egg volume was estimated by the equation volume Z 0.476 ! length ! breadth2 (Harris, 1964). Adult breeding great blackbacked gulls were caught at their nests using a nest trap trigged by a radio transmitter (Bustnes et al., 2001a). In all nests the eggs were replaced by dummies, and the incubated eggs were kept warm in a plastic box during catching and handling of birds. All birds were ringed by a numbered steel band and a coded polyvinyl chloride color band. The blood sample, 10–11 ml was taken from the wing vein and the samples were frozen in ÿ20 C for later analyses. The biometrics taken from each bird included: the head C bill length; the bill length; the bill height; the tarsus length; and the wing length using standard methods. Head and bill measurement was chosen to represent body size (Coulson and Thomas, 1983). The body mass was measured with a Pesola spring balance and the bird’s body mass was determined to the nearest 10 g. The same person took all measurements in order to minimize measuring errors. All birds were caught a minimum of 10 days after start of laying to ensure that they were incubating, and the nests were checked one to three times during the incubation period until hatching to estimate the rate of egg predation. In gulls the males are normally larger than females (Coulson and Thomas, 1983). Even though only one bird from each pair was caught the sex determination of the birds was possible after catching by comparing the colorringed bird with the other bird in the pair by telescope. The males were assumed to be larger, had a different head profile with more prominent eyebrow and relatively larger bill, compared to females. When field observations were compared to the linear measurements, there was no overlap in head C bill length or bill height between males and females (see also Mawhinney and Diamond, 1999a).
477
2.3. Chemical analyses The OC analyses were carried out at the Environmental Toxicology Laboratory at the Norwegian School of Veterinary Science/National Veterinary Institute. Samples of whole blood (ca 8 g) were weighed, and an internal standard (PCB-29 and PCB-112) was added. The methods used for extraction (cyclohexane and acetone), clean-up (with sulphuric acid) and quantification (gas chromatographic) of the samples were first described in Brevik (1978), and modifications are described in Andersen et al. (2001). The compounds analyzed were hexachlorobenzene (HCB), b-hexachlorocyclohexane (b-HCH), p,p#-dichlorodiphenyldichloroethylene (DDE), oxychlordane and the polychlorinated biphenyl (PCB) congeners: 28, 101, 99, 118, 138, 153, 170 and 180 (IUPAC [International Union for Pure and Applied Chemistry] numbers; Ballschmiter and Zell, 1980). Detection limits for individual compounds were determined as three times the noise level, and were found to be between 0.01 and 0.07 ng gÿ1 wet weight. The quantification was performed using the internal standards in each sample. The recovery percent varied from 95 to 112 (N Z 8), which are within the acceptable range set by the laboratory’s own reference system. The concentration in blood (wet mass) was used as a measure of OC concentration. This is considered as the most relevant method for studying potential toxic effects (Klaasen and Eaton, 1991; Bignert et al., 1993; Henriksen et al., 1996, 1998). In the closely related glaucous gull, recent studies suggest a relatively high short and long term stability of persistent OC levels (Henriksen et al., 1998; Bustnes et al., 2001b). Also for the great black-backed gull blood concentration seems to be a reliable measurement of the OC burden for an individual relative to other individuals in the population (Bustnes et al., in press). The correlations between all persistent PCB congeners (PCB-99, 118, 138, 153, 170 and 180) and their sum were extremely high (0.98–1.00, Table 1). Accordingly the sum of the persistent PCBs was chosen to represent PCBs (denoted: persistent PCBs) because no more information could be extracted from analyses of each congener in relation to reproductive parameters. The more volatile PCB-28 and -101 was also strongly correlated to the sum of persistent PCBs, but to a lesser extent than the persistent group (Table 1), and were together with HCB, b-HCH, oxychlordane and DDE analyzed separately for possible relationships with reproductive parameters. 2.4. Statistical analyses Females and males were analyzed separately to unravel potential sex-specific effects on reproductive
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Both males (F1,15 Z 20.7, P Z 0.0004, R2 Z 0.58) and females (F1,16 Z 7.75, P Z 0.013, R2 Z 0.33) caught early in the incubation period were heavier compared to birds caught late in the incubation period (Fig. 3). There was a significant negative relationship between body condition (body mass controlled for body size: head and bill) and all OCs except oxychlordane in females (Table 2), but not in males (P values between 0.15 and 0.68). We therefore controlled for body condition in all statistical models testing the relationship between reproductive parameters and OCs. 3.3. Reproductive variables 3.3.1. Egg laying date We controlled for incubation stage in all models testing the relationships between different OCs and egg 50 Females Males 40
30
20
3.1. OC levels in males and females There were significantly positive relationships between OC levels in females and the day in the incubation
PCB-180
PCB-170
3. Results
PCB-138
0
PCB-153
10
PCB-118
parameters. One outlier measurement of b-HCH level in the blood of one female was removed, based on evaluation of studentized residuals (Schlotzhauer and Littell, 1991). We did not adjust alpha-values for multiple comparisons according to Rothman (1990). Such adjustments could remove possible significant findings concerning ecological effects of different compounds. All P values are reported from two-tailed tests, and standard error is given for all means and estimates. We chose the best statistical models by backward selection, in which a reproductive variable was the dependent variable, and concentrations of OCs and potential confounding variables were independent variables. We started with a full model, and removed independent variables one at a time if they were not significant (P ! 0.05). Probability of nest predation and clutch size (2 or 3 eggs) in relation to OC levels was calculated using logistic regressions with a binary response variable (PROC GENMOD). Relationships between egg volume and OCs were analyzed using the mixed procedure (PROC MIXED, Type 1 test) in SAS, version 8e. Because first (a), second (b) and third (c) eggs in a clutch all came from one female, female was used as a random factor in the models to overcome the problem with pseudoreplication (SAS Inc, 1999). All OC values were log10 transformed to approximate normal distribution, while body measurements and body mass did not differ from normal distribution (Shapiro–Wilks test).
3.2. OCs and body condition
PCB-99
For example in females the correlation coefficient between the sum of persistent PCBs and HCB was 0.87. Data from Loppa Island, 2001.
PCB-101
0.75 0.59 0.62 0.79 0.82 0.93 0.98 0.98 1.00 1.00 0.99 0.99
PCB-28
0.87 0.82 0.75 0.88 0.89 0.93 0.98 0.97 1.00 0.99 0.99 0.98
DDE
HCB b-HCH Oxychlordane DDE PCB-28 PCB-101 PCB-99 PCB-118 PCB-153 PCB-138 PCB-180 PCB-170
Oxychlordan
Males (N Z 29)
ß-HCH
Females (N Z 23)
HBC
Compound
period when they were caught (hereafter incubation stage) for HCB (F1,16 Z 5.6, R2 Z 0.26, P Z 0.03), bHCH (F1,15 Z 8.7, R2 Z 0.37, P Z 0.01), DDE (F1,16 Z 6.1, R2 Z 0.28, P Z 0.024), near-significant for oxychlordane (F1,16 Z 4.3, R2 Z 0.21, P Z 0.053), PCB-28 (F1,16 Z 3.5, R2 Z 0.18, P Z 0.08), and the persistent PCBs (F1,16 Z 3.0, R2 Z 0.16, P Z 0.1), but not for PCB-101 (F1,16 Z 3.5, R2 Z 0.04, P Z 0.44). There were no relationship between incubation stage and OC levels for males (P values from 0.32 to 0.99). There were no differences in OC levels between males and females in a model that controlled for incubation stage (P values from 0.22 to 0.74), or in models that did not control for incubation stage (P values from 0.5 to 0.77, Fig. 2).
Concentration in blood (ng g-1, w.w.)
Table 1 The correlation coefficients (R-values) between the sum of persistent PCB congeners (PCB-99, 118, 153, 138, 180, and 170) and each organochlorine measured in the blood of great black-backed gulls
Contaminants Fig. 2. Organochlorine concentrations in the blood of female (N Z 23) and male (N Z 29) great black-backed gulls. All data from Loppa Island, 2001.
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Table 3 Relationships between egg laying date and the blood concentrations of different OCs, controlled for incubation stage, in female great blackbacked gulls
2.2 Males Females
Body mass (kg)
2.0
Day of egg laying
df
F
P
Estimate
SE
1.8
HCB Incubation stage
2, 15 2, 15
21.48 8.49
0.0003 0.01
11.29 ÿ0.48
2.44 0.17
1.6
b-HCH Incubation stage
2, 14 2, 14
8.02 3.93
0.01 0.067
15.26 ÿ0.49
5.39 0.25
1.4
Oxychlordane Incubation stage
2, 15 2, 15
10.91 4.4
0.048 0.063
8.8 ÿ0.38
2.66 0.19
DDE Incubation stage
2, 15 2, 15
14.8 6.38
0.0016 0.023
10.8 ÿ0.47
2.8 0.19
PCB-28 Incubation stage
2, 15 2, 15
9.09 3.07
0.0087 0.1
11.47 ÿ0.34
3.8 0.19
PCB-101 Incubation stage
2, 15 2, 15
13.06 1.56
0.0026 0.23
10.43 ÿ0.2
2.89 0.17
Persistent PCBs Incubation stage
2, 15 2, 15
9.1 2.8
0.0087 0.11
8.73 ÿ0.32
2.89 0.19
1.2
1.0 10
15
20
25
30
Incubation stage Fig. 3. Relationship between body condition and incubation stage (day in incubation period when captured) for male (N Z 29) and female (N Z 23) great black-backed gulls. Body size was controlled for in both sexes, but did not have any significant effects. All data from Loppa Island, 2001.
laying date. For females there was a strong positive correlation between laying date and all OCs measured (Table 3), and HCB was the strongest predictor (P Z 0.0003, Fig. 4). Neither body mass controlled for body size, clutch size or egg size showed significant relationships with laying date in females, neither alone (P values from 0.32 to 0.91) nor in the models with the different OCs (P values from 0.15 to 0.92). There were
One outlier measurement of b-HCH is removed. Persistent PCBs consist of congeners (IUPAC numbers): 99, 118, 153, 138, 170 and 180. All data from Loppa Island, 2001.
no significant relationships between egg laying date and blood levels of any OCs in males (P values from 0.13 to 0.37).
3.3.2. Clutch size Nests where birds were caught included 12 clutches with two eggs and 39 clutches with three eggs. In two 25
Table 2 The relationships between body mass (controlled for body size) and OC levels in female great black-backed gulls df
F
P
Estimate
SE
Body size HCB
2, 20 2, 20
24.01 6.12
!0.0001 0.023
0.025 ÿ0.11
0.005 0.045
Body size b-HCH
2, 19 2, 19
21.43 4.71
0.0002 0.043
0.025 ÿ0.14
0.005 0.066
Body size Oxychlordane
2, 20 2, 20
25.7 2.42
!0.0001 0.14
0.027
0.005
Body size DDE
2, 20 2, 20
22.12 5.29
0.0001 0.03
0.025 ÿ0.1
0.005 0.044
Body size PCB-28
2, 20 2, 20
21.87 4.89
0.0001 0.039
0.025 ÿ0.13
0.005 0.06
Body size PCB-101
2, 20 2, 20
14.25 10.8
0.0012 0.037
0.02 ÿ0.18
0.005 0.055
Body size Persistent PCBs
2, 20 2, 20
14.6 14.17
0.0011 0.012
0.02 ÿ0.17
0.005 0.044
Estimates and standard error are given for all significant effects (P ! 0.05). One outlier measurement of b-HCH in one female is removed. Persistent PCBs consist of congeners (IUPAC numbers): 99, 118, 153, 138, 170 and 180. All data from Loppa Island, 2001.
15
Laying date
Body mass
20
10
5
0
0
1
2
Log10 HCB level in blood Fig. 4. Relationship between egg laying date for the first egg in clutch and HCB level in the blood of female great black-backed gulls. In the final models, the effects of OCs are adjusted for incubation stage (Table 3). All data from Loppa Island, 2001.
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egg clutches the laying date was only known for two nests where males were caught and two nests where females were caught. Thus, laying date was not controlled for in the analyses of relationships between clutch size and OC levels. There were no significant relationships between clutch size and any of the OCs, neither in females (P values from 0.24 to 0.85) nor in males (P values from 0.17 to 0.73). Body size (females P Z 0.98, males P Z 0.17) and body mass (females P Z 0.52, males P Z 0.09) did not have significant effect on clutch size. 3.3.3. Clutch volume, egg volume and OCs For 15 clutches where the females were caught, the positions of the eggs in the laying sequence were known, all of which had three eggs. There were no significant relationships between total clutch volume and female blood concentration of any of the OCs (P values from 0.11 to 0.52). Body size and body mass had no effect in the models. We then tested the relationship between egg volume, controlled for position in the laying sequence (first, second or third egg laid) and the OC levels in the females (Table 4). There was a strong interaction between egg position in the sequence and all OCs except b-HCH and oxychlordane (Table 4). Body size was controlled in all models since it had a positive effect on egg volume, while all OCs had negative effects (Table 4). The significant interaction between OCs and egg volume
indicate that females with high OC levels, especially persistent PCBs, produce relatively smaller second and third eggs, compared to females with low levels of OCs (see Fig. 5 for persistent PCBs). 3.3.4. Nest predation The nest predation from both Raven Corvus corax and Hooded Crow Corvus cornix was very high, and of 111 nests included in this study 80 (71%) were predated. In nests where no birds were caught 47 of 59 (80%) nests were predated. In the remaining nests only one bird in each nest was caught, and the nest predation was 14 of 23 (60%) for nests where females were caught, and 18 of 29 (62%) for nests where males were caught. The probability of nest predation for both females and males was not significantly related to the date of egg laying, body mass, body size, clutch size or egg size (P values from 0.26 to 0.87). However, females with high OC levels had a higher risk of being predated. This was significant for b-HCH (c21,21 Z 5.0, P Z 0.025, one outlier removed), oxychlordane (c21,22 Z 4.15, P Z 0.041), and DDE (c21,22 Z 3.92, P Z 0.0478), nearsignificant for PCB-28 (c21,22 Z 3.56, P Z 0.059) and HCB (c21,22 Z 3.82, P Z 0.082), but not for the other OCs (P values from 0.19 to 0.53). There were no significant relationships between probability of nest predation for males and any of the OC levels (P values from 0.49 to 0.98).
Table 4 Relationships between egg volume and the interaction between egg number (1, 2 or 3) in the laying sequence and different OCs in blood of female great black-backed gulls, controlled for body size (Mixed Procedure in SAS, Type 1 test for fixed effects) df
Body size Egg number HCB HCB ! Egg number
1, 2, 1, 2,
Body size Egg number DDE DDE ! Egg number
F
P
26 26 26 26
9.91 23.17 0.31 4.73
0.0041 !0.0001 0.58 0.0177
1, 2, 1, 2,
26 26 26 26
9.85 23.80 0.251 5.21
0.0042 !0.0001 0.62 0.0125
Body size Egg number PCB-28 PCB-28 ! Egg number
1, 2, 1, 2,
26 26 26 26
10.0 22.78 0.42 4.42
0.0040 !0.0001 0.52 0.0222
Body size Egg number PCB-101 PCB-101 ! Egg number
1, 2, 1, 2,
26 26 26 26
10.38 28.50 0.91 8.80
0.0034 !0.0001 0.35 0.0012
Body size Egg number Persistent PCBs Persistent PCBs ! Egg number
1, 2, 1, 2,
26 26 26 26
10.52 29.87 1.08 9.85
0.0032 !0.0001 0.31 0.0007
Persistent PCBs consist of congeners (IUPAC numbers): 99, 118, 153, 138, 170 and 180. All data from Loppa Island, 2001.
110
Mean egg volume (ml)
Egg volume
120
100
90
80 1
2
3
Egg number in laying sequence Fig. 5. The mean egg volume (and standard error) of female great black-backed gulls with ‘‘low’’ PCB levels (lower than mean level, N Z 8; black points,) and ‘‘high’’ PCB levels (higher than mean level, N Z 7; white points). Statistics in Table 4. All data from Loppa Island, 2001.
M. Helberg et al. / Environmental Pollution 134 (2005) 475–483
4. Discussion This study showed that female great black-backed gulls with high blood levels of OCs laid their eggs later, had higher probability of nest predation, and a larger degree of egg volume decline within the clutch, compared to females with lower levels. This suggests that despite relatively low OC levels compared to nearby Arctic areas, such compounds have the potential to reduce the reproductive output and influence population regulatory mechanisms of seabirds at the subarctic coast of northern Norway. However, it is difficult to establish whether any particular compound was more important in causing the negative effects since different compounds tended to be most strongly related the different effects: HCB and DDE levels were strong predictors of females’ egg laying date; b-HCH, oxychlordane and DDE were strongest associated with nest predation; and persistent PCBs were strongest related to egg size differences within clutches. Previous field and laboratory studies have attributed various effects to different compounds, but in most cases the biological pathways and toxicity of different compounds are poorly understood in their basic details. However, the compounds measured in this study are known to have a wide variety of toxic effects in birds, including immune toxicity, impaired development of embryos, abnormal behavior and increased mortality (McCarty and Secord, 1999; Hoffman et al., 1996; Blus, 1996; Wiemeyer, 1996; Grasman et al., 2000; Bustnes et al., 2003). It is also possible that additive or synergistic effects of the compounds are important in creating effects, but this cannot be elucidated in a limited field study like this. There are, however, some indications that HCB are particularly stressful in the glaucous gull (Bustnes et al., 2002). There were two aspects of the levels of OCs in the great black-backed gulls in this study that differ from the glaucous gulls at Bear Island 500 km to the north. Firstly, that the blood concentrations of all OCs were much lower. For example, the mean sum of persistent PCBs in males was 100 ppb compared to 605 ppb in glaucous gulls, and 102 versus 289 ppb in females (Bustnes et al., 2003). Secondly, females had similar OC levels as males. This may suggest that female great black-backed gulls had spent much of their endogenous fat reserves which would have released much of their OC burden into the blood (see Henriksen et al., 1998). Moreover, females caught late in the incubation period had higher OC levels compared to females caught early during incubation, and females in poor body condition had higher levels of OCs than those in good condition. There were also negative relationships between body condition and incubation stage both in males and females, suggesting that the birds were unable to uphold their body condition throughout the breeding season. Local uptake as an alternative explanation for the
481
higher OC levels late in the incubation period is not very likely since levels of environment contaminants are not higher in the Loppa area compared to the wintering areas in the North Sea. Furthermore, most studies of body mass changes in larger gulls suggest that these species have no general strategy of decreasing body mass during incubation (Sibly and McCleery, 1983; Hario et al., 1991; Alonso-Alvarez et al., 2002), including some data on the great black-backed gull (Mawhinney et al., 1999b; J. O. Bustnes, unpublished data). The changes in body condition suggest that there were poor feeding conditions at Loppa in 2001, which may have forced the birds to alter their behavior in order to increase the chance of survival (Robin et al., 1998), for example by increased feeding time at the cost of reduced incubation constancy (Bustnes et al., 2001a). The high nest predation rate at Loppa (60–70%) also indicates that birds were poorly motivated for breeding, because of poor feeding conditions. All three adverse effects that were related to blood levels of OCs could potentially result from poor body condition, but body condition had no significant effects in any of the statistical models, suggesting that the toxic properties of OCs were triggering the effects. However, food stress may be fundamental in creating the basis for adverse effects of OCs. For example, an experimental study of ringed turtle doves Streptopelia risoria has shown that the combination of food deficiency and DDE causes more severe breeding failure than the presence of only one of these depressors (Keith and Mitchell, 1993). Doves that experienced both food shortage and DDE produced fewer offspring than control groups exposed only to food shortage. The interaction between OCs and food stress may thus be a synergistic relationship. The factors negatively related to OCs in this study are all important predictors of reproductive success. Firstly, egg laying date is important since early breeders typically are having the highest fitness in birds (Klomp, 1970). However, we cannot completely rule out alternative explanations for this relationship, e.g. late breeders could winter in more polluted areas than the early breeders (Henny et al., 1996; Johnstone et al., 1996; Mora, 1997) or they may have lost their clutch and replaced it. Relaying a new clutch requires mobilizing body lipid reserves, in addition to feeding of lipid-rich food, which both may lead to increased blood concentrations of OCs. Secondly, egg size strongly relates to chick hatching weight and size, as well as growth rate, fledging weight and survival (e.g. Parsons, 1970; Lundberg and Vaisanen, 1979). Most gull species lay clutches of three eggs, and the third egg is typically smallest and hatches last (Parsons, 1970). The relative differences both in size and hatching asynchrony in the clutch contribute to chick survival, the largest chicks killing or out-compete the
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smallest one (Stenning, 1996). In this study females with high OC concentrations had both small second and third eggs, suggesting that this is a way of reducing their reproductive costs. Increasing size differences probably lead to a more pronounced size hierarchy within the clutch which further could reduce reproductive output since small chicks are more likely to die (Clark and Wilson, 1981; Magrath, 1990; Stenning, 1996). Thirdly, higher nest predation is probably the most serious fitness cost related to OCs in our study population. OCs, such as DDE, have previously been found to increase nest predation in Merlins Falco columbarius (Fox and Donald, 1980). In herring gulls, it has been shown that birds from a contaminated site heated their eggs less and decreased nest defense compared to reference sites (Fox et al., 1978). Glaucous gulls at Bear Island with high OC levels tend to increase the time absent from the nest (Bustnes et al., 2001a). Even though the OC levels are higher at Bear Island compared to Loppa, it is possible that OCs in combination with food shortage contributed to longer, or more frequent feeding trips. In conclusion, despite relatively low levels of OCs found in our study population, there were clear negative relationships between reproductive performance and OCs in great black-backed gulls. At much higher OC levels, similar effects have been found in glaucous gulls, but at Bear Island there have been no indications of food shortage as there was at Loppa. It is possible that the effects of OCs would have been less prominent in years with improved feeding conditions, since the birds would be in better body condition and may better buffer the negative effects of OCs.
Acknowledgements We are grateful to Anuschka Polder and her team for the analyzing the blood samples. The study was funded by the Norwegian Research Council.
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Relationships between reproductive performance and organochlorine contaminants in great black-backed gulls (Larus marinus) Morten Helberga,b, Jan Ove Bustnesb,*, Kjell Einar Erikstada,b, Kai Ove Kristiansena, Janneche Utne Skaarec a Faculty of Science, Department of Biology, University of Tromsø, 9037 Tromsø, Norway Norwegian Institute for Nature Research, Unit for Arctic Ecology, The Polar Environmental Centre, N-9296 Tromsø, Norway c National Veterinary Institute, P.O. Box 8156 Dep., N-0033 Oslo, Norway
b
Received 2 April 2004; accepted 8 September 2004
Elevated blood concentrations of organochlorine contaminants correlate with poor reproductive performance in female great black-backed gulls. Abstract The great black-backed gull Larus marinus is a top predator in subarctic and temperate marine ecosystems, and the aim of this study was to investigate if organochlorines (OCs) were related to reproductive performance in this species at the subarctic parts of the Norwegian Coast. We measured blood levels of various OCs in 53 breeding birds. The OC levels were relatively low compared to levels found in nearby arctic areas. In females, however, there was a significant positive relationship between blood concentrations of OCs, especially hexachlorobenzene (HCB) and, p,p#-dichlorodiphenyldichloroethylene (DDE), and egg laying date, and a positive relationship between the probability of nest predation and blood concentration of b-hexachlorocyclohexane (b-HCH), oxychlordane, and DDE. In females with high levels of OCs, especially persistent polychlorinated biphenyls (PCBs), there was also a decline in egg volume as egg laying progressed; i.e. the second and third laid egg were relatively smaller, compared to females with low OC levels. No relationships between reproductive parameters and OC levels were found in males. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Subarctic; POPs; Reproductive output; Egg size; Nest predation; Egg laying date
1. Introduction Factors that cause environmental change beyond the range that wildlife normally are adapted to are likely to affect population growth and maintenance. One such factor is anthropogenic pollutants such as
* Corresponding author. Tel.: C47 77 75 04 07; fax: C47 77 75 04 01. E-mail address: [email protected] (J.O. Bustnes). 0269-7491/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2004.09.006
organochlorines (OCs), e.g. PCBs and DDT, which are distributed in food webs globally, and are known to have a wide array of biochemical, physiological and ecological effects (Jones and de Voogt, 1999; Burger et al., 2001). Food is the major source of OCs and due to their lipophilicity and persistent nature they biomagnify to their highest levels and cause greatest effects in top predators (Jones and de Voogt, 1999). For example, in the marine food web seabirds such as gulls Larus sp. are top predators, and probably particularly sensitive to OCs because they accumulate lipid reserves during parts
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of the year (Walker, 1990). For more than three decades, gulls such as the herring gull Larus argentatus have been studied in relation to accumulation and effects of OCs, particularly in the highly industrialized Great Lake District in North America (e.g. Herbert et al., 1999). However, high concentrations of OCs are not only found in industrialized areas, and even in the remote Arctic long-transported OCs, especially PCBs and DDE, have been found in alarmingly high levels, especially in top predators such as polar bears Ursus maritimus and glaucous gulls Larus hyperboreus (e.g. Bogan and Bourne, 1972; Gabrielsen et al., 1995; Bernhoft et al., 1997). Recently, a series of studies of the glaucous gull at Bear Island (74 30# N, 19 01# E) in the Norwegian Arctic (Fig. 1) have found this population to be negatively affected by OCs, including impaired reproduction and lowered survival (Sagerup et al., 2000; Bustnes et al., 2001a, 2002, 2003, 2004). However, virtually nothing is known about potential effects of OCs in seabirds along the Norwegian Coast which stretches from the industrialized North Sea area up to subarctic areas (Fig. 1). Previous seabird studies in this area have only determined levels and composition of OCs from seabird eggs and individuals (e.g. Fimreite et al., 1980; Moksnes and Norheim, 1986; Barrett et al., 1996; Henriksen et al., 1996). The aim of this study was
N
Bear Island
500 km
Loppa
#
to document whether OCs were having similar effects on the subarctic parts of the Norwegian Coast as at Bear Island about 500 km further north. We studied the great black-backed gull Larus marinus since it has a similar ecology as the glaucous gull, i.e. it has an opportunistic feeding ecology and it can also be a fierce predator (Cramp and Simmons, 1983; Good, 1998). High levels of OCs have also been found in this species in some areas (Weseloh et al., 2002). In arctic and subarctic areas, most of the environmental pollutants are long-transported and originate from non-point sources. A common feature of longtransported contaminants is that the concentrations of the different compounds are highly correlated. Accordingly, individuals with high levels of, for example, PCBs have relatively high levels of all persistent congeners and of other persistent OCs, such as hexachlorobenzene (HCB), b-hexachlorocyclohexane (b-HCH), oxychlordane, and p,p#-dichlorodiphenyldichloroethylene (DDE) (Mora et al., 1993; Henriksen et al., 1998; Jones and de Voogt, 1999; Bustnes et al., 2001b). In field studies it is thus difficult to document specific effects of different contaminants or contaminant groups (Bustnes et al., 2003). It is possible that single compounds are responsible for the observed effects, but additive effects of subgroups of the contaminants, or synergistic and antagonistic effects, may also be important. However, the degree of correlation between different compounds varies; i.e. some individuals are having relatively high concentrations of some compounds but not others, and it may be possible to exploit this variation and test if some compounds are more often stronger predictors of negative effects than others (Bustnes et al., 2003). In this study we used this approach, and measured the concentrations of pesticides (HCB and b-HCH) or pesticide metabolites (DDE and oxychlordane) and eight PCB congeners (six persistent congeners) in the blood of great black-backed gulls and correlated these levels to various reproductive parameters, including egg laying date, nest predation, egg size and clutch size.
2. Materials and methods
No
rw
ay
2.1. Study area
North Sea
Fig. 1. Map showing the study colony, Loppa Island, in relation to the Norwegian Arctic.
The study was carried out at Loppa Island (70 20# N, 21 24# E), Finnmark County, in northern Norway (Fig. 1). The island is 7.4 km long and 2.5 km wide and the highest peak (Rektind) is 274 m above sea level. This island provides a wide variety of habitats for breeding birds, varying from sheer cliffs at the western side, rocky shores at the northern and eastern side, and a sand-dune area in the southern part. The island has a plateau level that ranges from 100 m above sea level in the southern part, and rises up to 200 m in central parts and gradually
M. Helberg et al. / Environmental Pollution 134 (2005) 475–483
decreases down to 50 m above sea level in the northern end. In the western steep cliffs there are colonies of Atlantic puffins Fratercula arctica, razorbills Alca torda, and common murresUria aalge. The great black-backed gull breeds mostly in solitarily pairs or in loose colonies, sometimes with other gull species. In 2001 there were about 150 pairs breeding at Loppa, and the preferred habitat was at the plateau or around the rocky shores, and only a few pairs were breeding in the forest and lowland areas. 2.2. Methods From 27th of April and onwards all nests were checked daily if possible. The first eggs were found on 29th, and all eggs were marked with a marker pen and the nests were marked with a unique number. The axial length and breadth of each egg in the laying sequence (first, second and third eggs: designated egg a, b, and c) was measured with a digital caliper to the nearest 0.05 mm. Repeatability of egg volume was very high (df Z 267, R2 Z 0.9987, P ! 0.0001). The egg volume was estimated by the equation volume Z 0.476 ! length ! breadth2 (Harris, 1964). Adult breeding great blackbacked gulls were caught at their nests using a nest trap trigged by a radio transmitter (Bustnes et al., 2001a). In all nests the eggs were replaced by dummies, and the incubated eggs were kept warm in a plastic box during catching and handling of birds. All birds were ringed by a numbered steel band and a coded polyvinyl chloride color band. The blood sample, 10–11 ml was taken from the wing vein and the samples were frozen in ÿ20 C for later analyses. The biometrics taken from each bird included: the head C bill length; the bill length; the bill height; the tarsus length; and the wing length using standard methods. Head and bill measurement was chosen to represent body size (Coulson and Thomas, 1983). The body mass was measured with a Pesola spring balance and the bird’s body mass was determined to the nearest 10 g. The same person took all measurements in order to minimize measuring errors. All birds were caught a minimum of 10 days after start of laying to ensure that they were incubating, and the nests were checked one to three times during the incubation period until hatching to estimate the rate of egg predation. In gulls the males are normally larger than females (Coulson and Thomas, 1983). Even though only one bird from each pair was caught the sex determination of the birds was possible after catching by comparing the colorringed bird with the other bird in the pair by telescope. The males were assumed to be larger, had a different head profile with more prominent eyebrow and relatively larger bill, compared to females. When field observations were compared to the linear measurements, there was no overlap in head C bill length or bill height between males and females (see also Mawhinney and Diamond, 1999a).
477
2.3. Chemical analyses The OC analyses were carried out at the Environmental Toxicology Laboratory at the Norwegian School of Veterinary Science/National Veterinary Institute. Samples of whole blood (ca 8 g) were weighed, and an internal standard (PCB-29 and PCB-112) was added. The methods used for extraction (cyclohexane and acetone), clean-up (with sulphuric acid) and quantification (gas chromatographic) of the samples were first described in Brevik (1978), and modifications are described in Andersen et al. (2001). The compounds analyzed were hexachlorobenzene (HCB), b-hexachlorocyclohexane (b-HCH), p,p#-dichlorodiphenyldichloroethylene (DDE), oxychlordane and the polychlorinated biphenyl (PCB) congeners: 28, 101, 99, 118, 138, 153, 170 and 180 (IUPAC [International Union for Pure and Applied Chemistry] numbers; Ballschmiter and Zell, 1980). Detection limits for individual compounds were determined as three times the noise level, and were found to be between 0.01 and 0.07 ng gÿ1 wet weight. The quantification was performed using the internal standards in each sample. The recovery percent varied from 95 to 112 (N Z 8), which are within the acceptable range set by the laboratory’s own reference system. The concentration in blood (wet mass) was used as a measure of OC concentration. This is considered as the most relevant method for studying potential toxic effects (Klaasen and Eaton, 1991; Bignert et al., 1993; Henriksen et al., 1996, 1998). In the closely related glaucous gull, recent studies suggest a relatively high short and long term stability of persistent OC levels (Henriksen et al., 1998; Bustnes et al., 2001b). Also for the great black-backed gull blood concentration seems to be a reliable measurement of the OC burden for an individual relative to other individuals in the population (Bustnes et al., in press). The correlations between all persistent PCB congeners (PCB-99, 118, 138, 153, 170 and 180) and their sum were extremely high (0.98–1.00, Table 1). Accordingly the sum of the persistent PCBs was chosen to represent PCBs (denoted: persistent PCBs) because no more information could be extracted from analyses of each congener in relation to reproductive parameters. The more volatile PCB-28 and -101 was also strongly correlated to the sum of persistent PCBs, but to a lesser extent than the persistent group (Table 1), and were together with HCB, b-HCH, oxychlordane and DDE analyzed separately for possible relationships with reproductive parameters. 2.4. Statistical analyses Females and males were analyzed separately to unravel potential sex-specific effects on reproductive
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Both males (F1,15 Z 20.7, P Z 0.0004, R2 Z 0.58) and females (F1,16 Z 7.75, P Z 0.013, R2 Z 0.33) caught early in the incubation period were heavier compared to birds caught late in the incubation period (Fig. 3). There was a significant negative relationship between body condition (body mass controlled for body size: head and bill) and all OCs except oxychlordane in females (Table 2), but not in males (P values between 0.15 and 0.68). We therefore controlled for body condition in all statistical models testing the relationship between reproductive parameters and OCs. 3.3. Reproductive variables 3.3.1. Egg laying date We controlled for incubation stage in all models testing the relationships between different OCs and egg 50 Females Males 40
30
20
3.1. OC levels in males and females There were significantly positive relationships between OC levels in females and the day in the incubation
PCB-180
PCB-170
3. Results
PCB-138
0
PCB-153
10
PCB-118
parameters. One outlier measurement of b-HCH level in the blood of one female was removed, based on evaluation of studentized residuals (Schlotzhauer and Littell, 1991). We did not adjust alpha-values for multiple comparisons according to Rothman (1990). Such adjustments could remove possible significant findings concerning ecological effects of different compounds. All P values are reported from two-tailed tests, and standard error is given for all means and estimates. We chose the best statistical models by backward selection, in which a reproductive variable was the dependent variable, and concentrations of OCs and potential confounding variables were independent variables. We started with a full model, and removed independent variables one at a time if they were not significant (P ! 0.05). Probability of nest predation and clutch size (2 or 3 eggs) in relation to OC levels was calculated using logistic regressions with a binary response variable (PROC GENMOD). Relationships between egg volume and OCs were analyzed using the mixed procedure (PROC MIXED, Type 1 test) in SAS, version 8e. Because first (a), second (b) and third (c) eggs in a clutch all came from one female, female was used as a random factor in the models to overcome the problem with pseudoreplication (SAS Inc, 1999). All OC values were log10 transformed to approximate normal distribution, while body measurements and body mass did not differ from normal distribution (Shapiro–Wilks test).
3.2. OCs and body condition
PCB-99
For example in females the correlation coefficient between the sum of persistent PCBs and HCB was 0.87. Data from Loppa Island, 2001.
PCB-101
0.75 0.59 0.62 0.79 0.82 0.93 0.98 0.98 1.00 1.00 0.99 0.99
PCB-28
0.87 0.82 0.75 0.88 0.89 0.93 0.98 0.97 1.00 0.99 0.99 0.98
DDE
HCB b-HCH Oxychlordane DDE PCB-28 PCB-101 PCB-99 PCB-118 PCB-153 PCB-138 PCB-180 PCB-170
Oxychlordan
Males (N Z 29)
ß-HCH
Females (N Z 23)
HBC
Compound
period when they were caught (hereafter incubation stage) for HCB (F1,16 Z 5.6, R2 Z 0.26, P Z 0.03), bHCH (F1,15 Z 8.7, R2 Z 0.37, P Z 0.01), DDE (F1,16 Z 6.1, R2 Z 0.28, P Z 0.024), near-significant for oxychlordane (F1,16 Z 4.3, R2 Z 0.21, P Z 0.053), PCB-28 (F1,16 Z 3.5, R2 Z 0.18, P Z 0.08), and the persistent PCBs (F1,16 Z 3.0, R2 Z 0.16, P Z 0.1), but not for PCB-101 (F1,16 Z 3.5, R2 Z 0.04, P Z 0.44). There were no relationship between incubation stage and OC levels for males (P values from 0.32 to 0.99). There were no differences in OC levels between males and females in a model that controlled for incubation stage (P values from 0.22 to 0.74), or in models that did not control for incubation stage (P values from 0.5 to 0.77, Fig. 2).
Concentration in blood (ng g-1, w.w.)
Table 1 The correlation coefficients (R-values) between the sum of persistent PCB congeners (PCB-99, 118, 153, 138, 180, and 170) and each organochlorine measured in the blood of great black-backed gulls
Contaminants Fig. 2. Organochlorine concentrations in the blood of female (N Z 23) and male (N Z 29) great black-backed gulls. All data from Loppa Island, 2001.
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Table 3 Relationships between egg laying date and the blood concentrations of different OCs, controlled for incubation stage, in female great blackbacked gulls
2.2 Males Females
Body mass (kg)
2.0
Day of egg laying
df
F
P
Estimate
SE
1.8
HCB Incubation stage
2, 15 2, 15
21.48 8.49
0.0003 0.01
11.29 ÿ0.48
2.44 0.17
1.6
b-HCH Incubation stage
2, 14 2, 14
8.02 3.93
0.01 0.067
15.26 ÿ0.49
5.39 0.25
1.4
Oxychlordane Incubation stage
2, 15 2, 15
10.91 4.4
0.048 0.063
8.8 ÿ0.38
2.66 0.19
DDE Incubation stage
2, 15 2, 15
14.8 6.38
0.0016 0.023
10.8 ÿ0.47
2.8 0.19
PCB-28 Incubation stage
2, 15 2, 15
9.09 3.07
0.0087 0.1
11.47 ÿ0.34
3.8 0.19
PCB-101 Incubation stage
2, 15 2, 15
13.06 1.56
0.0026 0.23
10.43 ÿ0.2
2.89 0.17
Persistent PCBs Incubation stage
2, 15 2, 15
9.1 2.8
0.0087 0.11
8.73 ÿ0.32
2.89 0.19
1.2
1.0 10
15
20
25
30
Incubation stage Fig. 3. Relationship between body condition and incubation stage (day in incubation period when captured) for male (N Z 29) and female (N Z 23) great black-backed gulls. Body size was controlled for in both sexes, but did not have any significant effects. All data from Loppa Island, 2001.
laying date. For females there was a strong positive correlation between laying date and all OCs measured (Table 3), and HCB was the strongest predictor (P Z 0.0003, Fig. 4). Neither body mass controlled for body size, clutch size or egg size showed significant relationships with laying date in females, neither alone (P values from 0.32 to 0.91) nor in the models with the different OCs (P values from 0.15 to 0.92). There were
One outlier measurement of b-HCH is removed. Persistent PCBs consist of congeners (IUPAC numbers): 99, 118, 153, 138, 170 and 180. All data from Loppa Island, 2001.
no significant relationships between egg laying date and blood levels of any OCs in males (P values from 0.13 to 0.37).
3.3.2. Clutch size Nests where birds were caught included 12 clutches with two eggs and 39 clutches with three eggs. In two 25
Table 2 The relationships between body mass (controlled for body size) and OC levels in female great black-backed gulls df
F
P
Estimate
SE
Body size HCB
2, 20 2, 20
24.01 6.12
!0.0001 0.023
0.025 ÿ0.11
0.005 0.045
Body size b-HCH
2, 19 2, 19
21.43 4.71
0.0002 0.043
0.025 ÿ0.14
0.005 0.066
Body size Oxychlordane
2, 20 2, 20
25.7 2.42
!0.0001 0.14
0.027
0.005
Body size DDE
2, 20 2, 20
22.12 5.29
0.0001 0.03
0.025 ÿ0.1
0.005 0.044
Body size PCB-28
2, 20 2, 20
21.87 4.89
0.0001 0.039
0.025 ÿ0.13
0.005 0.06
Body size PCB-101
2, 20 2, 20
14.25 10.8
0.0012 0.037
0.02 ÿ0.18
0.005 0.055
Body size Persistent PCBs
2, 20 2, 20
14.6 14.17
0.0011 0.012
0.02 ÿ0.17
0.005 0.044
Estimates and standard error are given for all significant effects (P ! 0.05). One outlier measurement of b-HCH in one female is removed. Persistent PCBs consist of congeners (IUPAC numbers): 99, 118, 153, 138, 170 and 180. All data from Loppa Island, 2001.
15
Laying date
Body mass
20
10
5
0
0
1
2
Log10 HCB level in blood Fig. 4. Relationship between egg laying date for the first egg in clutch and HCB level in the blood of female great black-backed gulls. In the final models, the effects of OCs are adjusted for incubation stage (Table 3). All data from Loppa Island, 2001.
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M. Helberg et al. / Environmental Pollution 134 (2005) 475–483
egg clutches the laying date was only known for two nests where males were caught and two nests where females were caught. Thus, laying date was not controlled for in the analyses of relationships between clutch size and OC levels. There were no significant relationships between clutch size and any of the OCs, neither in females (P values from 0.24 to 0.85) nor in males (P values from 0.17 to 0.73). Body size (females P Z 0.98, males P Z 0.17) and body mass (females P Z 0.52, males P Z 0.09) did not have significant effect on clutch size. 3.3.3. Clutch volume, egg volume and OCs For 15 clutches where the females were caught, the positions of the eggs in the laying sequence were known, all of which had three eggs. There were no significant relationships between total clutch volume and female blood concentration of any of the OCs (P values from 0.11 to 0.52). Body size and body mass had no effect in the models. We then tested the relationship between egg volume, controlled for position in the laying sequence (first, second or third egg laid) and the OC levels in the females (Table 4). There was a strong interaction between egg position in the sequence and all OCs except b-HCH and oxychlordane (Table 4). Body size was controlled in all models since it had a positive effect on egg volume, while all OCs had negative effects (Table 4). The significant interaction between OCs and egg volume
indicate that females with high OC levels, especially persistent PCBs, produce relatively smaller second and third eggs, compared to females with low levels of OCs (see Fig. 5 for persistent PCBs). 3.3.4. Nest predation The nest predation from both Raven Corvus corax and Hooded Crow Corvus cornix was very high, and of 111 nests included in this study 80 (71%) were predated. In nests where no birds were caught 47 of 59 (80%) nests were predated. In the remaining nests only one bird in each nest was caught, and the nest predation was 14 of 23 (60%) for nests where females were caught, and 18 of 29 (62%) for nests where males were caught. The probability of nest predation for both females and males was not significantly related to the date of egg laying, body mass, body size, clutch size or egg size (P values from 0.26 to 0.87). However, females with high OC levels had a higher risk of being predated. This was significant for b-HCH (c21,21 Z 5.0, P Z 0.025, one outlier removed), oxychlordane (c21,22 Z 4.15, P Z 0.041), and DDE (c21,22 Z 3.92, P Z 0.0478), nearsignificant for PCB-28 (c21,22 Z 3.56, P Z 0.059) and HCB (c21,22 Z 3.82, P Z 0.082), but not for the other OCs (P values from 0.19 to 0.53). There were no significant relationships between probability of nest predation for males and any of the OC levels (P values from 0.49 to 0.98).
Table 4 Relationships between egg volume and the interaction between egg number (1, 2 or 3) in the laying sequence and different OCs in blood of female great black-backed gulls, controlled for body size (Mixed Procedure in SAS, Type 1 test for fixed effects) df
Body size Egg number HCB HCB ! Egg number
1, 2, 1, 2,
Body size Egg number DDE DDE ! Egg number
F
P
26 26 26 26
9.91 23.17 0.31 4.73
0.0041 !0.0001 0.58 0.0177
1, 2, 1, 2,
26 26 26 26
9.85 23.80 0.251 5.21
0.0042 !0.0001 0.62 0.0125
Body size Egg number PCB-28 PCB-28 ! Egg number
1, 2, 1, 2,
26 26 26 26
10.0 22.78 0.42 4.42
0.0040 !0.0001 0.52 0.0222
Body size Egg number PCB-101 PCB-101 ! Egg number
1, 2, 1, 2,
26 26 26 26
10.38 28.50 0.91 8.80
0.0034 !0.0001 0.35 0.0012
Body size Egg number Persistent PCBs Persistent PCBs ! Egg number
1, 2, 1, 2,
26 26 26 26
10.52 29.87 1.08 9.85
0.0032 !0.0001 0.31 0.0007
Persistent PCBs consist of congeners (IUPAC numbers): 99, 118, 153, 138, 170 and 180. All data from Loppa Island, 2001.
110
Mean egg volume (ml)
Egg volume
120
100
90
80 1
2
3
Egg number in laying sequence Fig. 5. The mean egg volume (and standard error) of female great black-backed gulls with ‘‘low’’ PCB levels (lower than mean level, N Z 8; black points,) and ‘‘high’’ PCB levels (higher than mean level, N Z 7; white points). Statistics in Table 4. All data from Loppa Island, 2001.
M. Helberg et al. / Environmental Pollution 134 (2005) 475–483
4. Discussion This study showed that female great black-backed gulls with high blood levels of OCs laid their eggs later, had higher probability of nest predation, and a larger degree of egg volume decline within the clutch, compared to females with lower levels. This suggests that despite relatively low OC levels compared to nearby Arctic areas, such compounds have the potential to reduce the reproductive output and influence population regulatory mechanisms of seabirds at the subarctic coast of northern Norway. However, it is difficult to establish whether any particular compound was more important in causing the negative effects since different compounds tended to be most strongly related the different effects: HCB and DDE levels were strong predictors of females’ egg laying date; b-HCH, oxychlordane and DDE were strongest associated with nest predation; and persistent PCBs were strongest related to egg size differences within clutches. Previous field and laboratory studies have attributed various effects to different compounds, but in most cases the biological pathways and toxicity of different compounds are poorly understood in their basic details. However, the compounds measured in this study are known to have a wide variety of toxic effects in birds, including immune toxicity, impaired development of embryos, abnormal behavior and increased mortality (McCarty and Secord, 1999; Hoffman et al., 1996; Blus, 1996; Wiemeyer, 1996; Grasman et al., 2000; Bustnes et al., 2003). It is also possible that additive or synergistic effects of the compounds are important in creating effects, but this cannot be elucidated in a limited field study like this. There are, however, some indications that HCB are particularly stressful in the glaucous gull (Bustnes et al., 2002). There were two aspects of the levels of OCs in the great black-backed gulls in this study that differ from the glaucous gulls at Bear Island 500 km to the north. Firstly, that the blood concentrations of all OCs were much lower. For example, the mean sum of persistent PCBs in males was 100 ppb compared to 605 ppb in glaucous gulls, and 102 versus 289 ppb in females (Bustnes et al., 2003). Secondly, females had similar OC levels as males. This may suggest that female great black-backed gulls had spent much of their endogenous fat reserves which would have released much of their OC burden into the blood (see Henriksen et al., 1998). Moreover, females caught late in the incubation period had higher OC levels compared to females caught early during incubation, and females in poor body condition had higher levels of OCs than those in good condition. There were also negative relationships between body condition and incubation stage both in males and females, suggesting that the birds were unable to uphold their body condition throughout the breeding season. Local uptake as an alternative explanation for the
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higher OC levels late in the incubation period is not very likely since levels of environment contaminants are not higher in the Loppa area compared to the wintering areas in the North Sea. Furthermore, most studies of body mass changes in larger gulls suggest that these species have no general strategy of decreasing body mass during incubation (Sibly and McCleery, 1983; Hario et al., 1991; Alonso-Alvarez et al., 2002), including some data on the great black-backed gull (Mawhinney et al., 1999b; J. O. Bustnes, unpublished data). The changes in body condition suggest that there were poor feeding conditions at Loppa in 2001, which may have forced the birds to alter their behavior in order to increase the chance of survival (Robin et al., 1998), for example by increased feeding time at the cost of reduced incubation constancy (Bustnes et al., 2001a). The high nest predation rate at Loppa (60–70%) also indicates that birds were poorly motivated for breeding, because of poor feeding conditions. All three adverse effects that were related to blood levels of OCs could potentially result from poor body condition, but body condition had no significant effects in any of the statistical models, suggesting that the toxic properties of OCs were triggering the effects. However, food stress may be fundamental in creating the basis for adverse effects of OCs. For example, an experimental study of ringed turtle doves Streptopelia risoria has shown that the combination of food deficiency and DDE causes more severe breeding failure than the presence of only one of these depressors (Keith and Mitchell, 1993). Doves that experienced both food shortage and DDE produced fewer offspring than control groups exposed only to food shortage. The interaction between OCs and food stress may thus be a synergistic relationship. The factors negatively related to OCs in this study are all important predictors of reproductive success. Firstly, egg laying date is important since early breeders typically are having the highest fitness in birds (Klomp, 1970). However, we cannot completely rule out alternative explanations for this relationship, e.g. late breeders could winter in more polluted areas than the early breeders (Henny et al., 1996; Johnstone et al., 1996; Mora, 1997) or they may have lost their clutch and replaced it. Relaying a new clutch requires mobilizing body lipid reserves, in addition to feeding of lipid-rich food, which both may lead to increased blood concentrations of OCs. Secondly, egg size strongly relates to chick hatching weight and size, as well as growth rate, fledging weight and survival (e.g. Parsons, 1970; Lundberg and Vaisanen, 1979). Most gull species lay clutches of three eggs, and the third egg is typically smallest and hatches last (Parsons, 1970). The relative differences both in size and hatching asynchrony in the clutch contribute to chick survival, the largest chicks killing or out-compete the
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smallest one (Stenning, 1996). In this study females with high OC concentrations had both small second and third eggs, suggesting that this is a way of reducing their reproductive costs. Increasing size differences probably lead to a more pronounced size hierarchy within the clutch which further could reduce reproductive output since small chicks are more likely to die (Clark and Wilson, 1981; Magrath, 1990; Stenning, 1996). Thirdly, higher nest predation is probably the most serious fitness cost related to OCs in our study population. OCs, such as DDE, have previously been found to increase nest predation in Merlins Falco columbarius (Fox and Donald, 1980). In herring gulls, it has been shown that birds from a contaminated site heated their eggs less and decreased nest defense compared to reference sites (Fox et al., 1978). Glaucous gulls at Bear Island with high OC levels tend to increase the time absent from the nest (Bustnes et al., 2001a). Even though the OC levels are higher at Bear Island compared to Loppa, it is possible that OCs in combination with food shortage contributed to longer, or more frequent feeding trips. In conclusion, despite relatively low levels of OCs found in our study population, there were clear negative relationships between reproductive performance and OCs in great black-backed gulls. At much higher OC levels, similar effects have been found in glaucous gulls, but at Bear Island there have been no indications of food shortage as there was at Loppa. It is possible that the effects of OCs would have been less prominent in years with improved feeding conditions, since the birds would be in better body condition and may better buffer the negative effects of OCs.
Acknowledgements We are grateful to Anuschka Polder and her team for the analyzing the blood samples. The study was funded by the Norwegian Research Council.
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