Reproductive rate as an index of the environment: 28 years of monitoring Weddell seals in east Antarctica.

Brief description

Publication of these results is currently in progress with the Journal of Animal Ecology.

Summary
1.An efficient method of describing change in Antarctic marine ecosystems is long-term monitoring of land-breeding marine predators. High-level predators are used to index the state of environment on the notion that perturbations in the ecosystem will affect their diet, reproductive performance and other demographics. For this purpose, Weddell seals breeding at the Vestfold Hills have been marked and re-sighted for the past 28 years (1973 - 2000).
2.Successful reproduction requires considerable energetic resources. The difference between rates of conception and rates of parturition suggests pregnant females abort reproductive attempts when their energy stores are low. In this way, annual rates of reproduction (i.e. parturition) are a measure of foraging efficiency.
3.Previous attempts to estimate Weddell seal reproduction have been biased by different rates of re-sighting breeding and non-breeding females. We used multistate mark and re-sight models to account for this and other variables when estimating reproductive rate.
4.The amplitude of temporal variation was much greater for reproduction than for survivorship, indicating that parous (breeding) females maximised survival by reproducing less. This strategy could be successful in fluctuating environments because seals live longer and experience more reproductive occasions.
5.The population had low reproductive rates from 1983 to 1985 and throughout the 1990s. In those years, potential recruitment into breeding groups was reduced to 50 - 60 % of the cohort before viable pups were even born.
6.Even in years of low reproductive rate, typically half (52%) of the breeding females produced pups. It seemed that individuals differed in their foraging success and thus body condition and / or their functional response to this.
7.There was no evidence for costs of reproduction. We infer that the seals responded to environmental conditions prior to parturition, as opposed to proceeding with reproduction when inadequately resourced and depleting energy resources such that they had lower probability of surviving or reproducing the following year.
8.Synthesis and applications: This study demonstrates a method of estimating reproductive rate that overcomes bias inherent in traditional methods. Estimated in this way, we propose that reproductive rate is the best indicator of the state of marine ecosystems that can be indexed for Weddell seals.

The fields in this dataset are:

Year
Standard Error
Upper confidence interval
Lower confidence interval
Breeding probability
Upper error bar
Lower error bar

Lineage

Progress Code: completed

Statement: These estimates are derived from models based on the Arnason-Schwarz method, and account for the probability of females surviving between samples; being resighted (and thus included in the sample); and being in breeding or non-breeding state.

In the base model, breeding probability follows a first-order Markov process in that the probability of breeding at time i+1 depends on breeding state at time i. Therefore, reproductive rate can be defined as the probability of being in breeding state at time i+1, conditional on surviving from time i to i+1.

The method accounts for many of the biases inherent in traditional estimates of reproductive rate since re-sight rates can vary between breeding states and survivorship is also incorporated.