Understanding the drivers underlying fluctuations in the size of animal populations is central to ecology, conservation biology, and wildlife management. Reliable estimates of survival probabilities are key to population viability assessments, and patterns of variation in survival can help inferring the causal factors behind detected changes in population size. We investigated whether variation in age‐ and sex‐specific survival probabilities could help explain the increasing trend in population size detected in a small, discrete population of bottlenose dolphins Tursiops truncatus off the east coast of Scotland. To estimate annual survival probabilities, we applied capture–recapture models to photoidentification data collected from 1989 to 2015. We used robust design models accounting for temporary emigration to estimate juvenile and adult survival, multistate models to estimate sex‐specific survival, and age models to estimate calf survival. We found strong support for an increase in juvenile/adult annual survival from 93.1% to 96.0% over the study period, most likely caused by a change in juvenile survival. Examination of sex‐specific variation showed weaker support for this trend being a result of increasing female survival, which was overall higher than for males and animals of unknown sex. Calf survival was lower in the first than second year; a bias in estimating third‐year survival will likely exist in similar studies. There was some support first‐born calf survival being lower than for calves born subsequently. Coastal marine mammal populations are subject to the impacts of environmental change, increasing anthropogenic disturbance and the effects of management measures. Survival estimates are essential to improve our understanding of population dynamics and help predict how future pressures may impact populations, but obtaining robust information on the life history of long‐lived species is challenging. Our study illustrates how knowledge of survival can be increased by applying a robust analytical framework to photoidentification data.

Abstract. Accurate estimates of fecundity rate are key to population assessments and effectively direct conservation efforts. We present a new approach to estimate fecundity rate based on the probability of a female giving birth, conditional on a previous birth t years ago, from which an expected inter-birth interval (IBI) can be estimated. We use generalized linear mixed-effects models to account for individual and temporal variability and apply the approach to individual reproductive histories of bottlenose dolphins (Tursiops truncatus) from the east coast of Scotland. We estimate a fecundity rate of 0.222 (95% CI = 0.218-0.253) and an expected IBI of 4.49 yr (95% CI = 3.94-4.93 yr). We use simulated data samples to show that the approach produces estimates with a minimum bias of <3%. Simulations are also used to investigate the effect of the most common data-driven biases in the estimates of birth intervals and fecundity rate; we recommend longitudinal studies of at least 10 yr and capture probabilities of at least 0.3 when using this methodology. The approach may be modified to incorporate other parameters of interest and should be applicable to any population with comprehensive data on birth intervals.

1. The efficacy of marine protected areas (MPAs) depends on clear conservation objectives and ecologically meaningful boundaries. The east coast of Scotland bottlenose dolphin population expanded its distributional range during the 1990s beyond the boundaries of the Moray Firth Special Area of Conservation (SAC), originally proposed to contain their core area of distribution. Two decades on, this study assesses the importance for this population of St Andrews Bay and the Tay estuary, 300 km south of the SAC. 2. Photoidentification data from 2009 to 2015 were analysed using mark-recapture models to investigate the proportion of the population that uses St Andrews Bay and the Tay estuary. Habitat models were fitted to bottlenose dolphin presenceabsence data to identify areas of high use. 3. The estimated number of dolphins using St Andrews Bay and the Tay estuary during the summer increased from 91 (95% confidence interval 78-106) in 2009 to 114 (95% confidence interval 95-137) in 2015, representing, on average, 52.5% of the total estimated east-coast population for that period. Spatial mixing of individuals during the summer between St Andrews Bay and the Tay estuary and the Moray Firth SAC was estimated to be a minimum of~6% per year and~30% over the study period. The entrance to the Firth of Tay and waters around Montrose were identified as areas of consistent high use. 4. The importance of St Andrews Bay and the Tay estuary reconfirms that effective monitoring of the population requires dedicated effort in both this area and the SAC. The results lead to consideration of the wider context of area-based management for the conservation/management of highly mobile wide-ranging species and human activities that might impact them.

Fostering and allo-suckling are widespread among pinnipeds, and several hypotheses have been formulated to explain their occurrence. Here, we describe the occurrence of allo-suckling in harbour seals from photo-identification data of females and pups in Orkney (Scotland) during the pupping seasons between 2016 and 2019. We used a generalised linear model framework to investigate the effect of allo-suckling on the duration of lactation (females) and of nursing period (pups). A generalised additive model framework was used to explore how the probability of allo-suckling varied throughout the pupping season, and with changes in mother-pup separation time. Allo-suckling was observed in 31 females, at higher rates (18–37% of lactating females and 18–47% of the pups every year) than those observed in other phocid populations, with 13 females allo-suckling in multiple years. The duration of the pups’ nursing period was not affected by allo-suckling occurrence. However, females in mother-pup pairs where both mother and pup allo-suckled had longer lactation duration than when only the pup allo-suckled, or than in pairs where no allo-suckling was observed. The probability of allo-suckling increased during the pupping season and with increased mother-pup separation time. However, the proximate causes and the consequences on future reproductive output and pup survival remain unknown.Significance statementAllo-suckling, where females nurse others’ young, is widespread in pinnipeds, particularly among true seals. Given the high costs of lactation in pinnipeds, allo-suckling is a puzzling behaviour. Using photo-identification and field observations, we examined the occurrence of allo-suckling in harbour seals at a colony in Orkney, Scotland. We found that allo-suckling is common among seals at the study site, and at rates higher than reported elsewhere. Our results show that allo-suckling does not appear to affect the duration of the pups’ nursing period but does increase the lactation duration of females who suckle other pups and whose own pups also allo-suckle. This study highlights an area which requires further investigation as the energetic costs and benefits of allo-suckling remain poorly understood.

Pregnancy status in harbour seals can be estimated from concentrations of progesterone in blubber as well as in blood samples, which are significantly higher in pregnant than non-pregnant animals. This study investigated the accuracy of estimating pregnancy rates using samples from live-captured and released harbour seals from three regions around Scotland, coupled with observed pregnancy outcomes. Concentrations of progesterone in blood (plasma) and blubber were obtained during the capture of animals early in the year (February to May). Individual animals were identified from the unique markings on their pelage, with a proportion (n=51) of females re-sighted during the subsequent breeding season and the reproductive outcomes determined (pregnant or possibly nonpregnant) during observations from long-term photo-identification studies. Generalised linear models with a binomial link function were fitted to training (60% of the data) and test datasets (40% of the data) to estimate pregnancy status from progesterone concentrations in blubber, plasma or both, and a received operating curves (ROC) approach was used to evaluate the performance of each classifier. The accuracy for the plasma concentrations was 85% with a high classification performance (as estimated from an area under the curve (AUC) of 0.82). The Youden method to determine the cut-point (threshold) and bootstrapping the training dataset resulted in a cut-point of 58 ng ml -1 (95 th percentiles, 25 -102 ng ml -1 ). For blubber, the accuracy was 77% (AUC=0.86) with an optimal cut-point of 56 ng g -1 (95 th percentiles, 26 -223 ng g -1 ). In the combined analysis (both blubber and plasma), the accuracy was 87.5% (AUC 0.81) with the cut-points of 72 ng ml -1 (95 th percentiles, 25 -103 ng ml -1 ) in plasma and 56 ng g -1 (95 th percentiles, 26 -223 ng g -1 ) in blubber. These thresholds were then used to estimate the pregnancy proportions among adult females at the three study sites, including those that were not included in the photo-id studies. Proportions were high at all sites, (63% -100%) regardless of which matrices were used and were not statistically significantly different from each other but suggested that analysing concentrations in both sample matrices would minimise the uncertainty.