Intra- versus inter-annual variation in counts of reef fishes and interpretations of long-term monitoring studies

Abstract

Many sampling strategies have been proposed as appropriate for describing spatial patterns in marine organisms. There remain, however, many problems with the description, analysis and interpretation of temporal variation in abundances of organisms. In particular, there is a need to understand temporal error in the estimation of abundance of mobile organisms. In this paper we report estimates of temporal variation in abundances of tropical reef fishes attributable to sampling error at diurnal, daily and 'monthly' scales and compare these to inter-annual variation that might arise from processes such as mortality and recruitment. Uncertainty in estimates taken from the same sites over consecutive days was large for several species and accounted for the majority of error in estimates of abundance within years. Sources of error in estimates of abundance are discussed with consideration of the implications for long-term sampling and monitoring of fish assemblages. Shortterm temporal variation must be considered along with spatial variation in the design and interpretation of temporal studies of mobile species. KEY WORDS: Reef fish···Visual census···Temporal errorResale or republication not permitted without written consent of the publisher

Responses of reef fish communities to coral declines on the Great Barrier Reef

“…J o h n B r e w e r R ib D ip G a n n e t C a y E a s t C a y Number of herbivore species . 'Stable' abundances represent changes < 25% that may be due to sampling error (Thompson & Mapstone 1997). Increases or decreases in abundance represent changes > 25%…”

“…Species percent changes are ranked from largest increases to largest decreases along the x-axis. The area bounded by the light grey lines represents increases or decreases of < 25% that may be due to sampling error (Thompson & Mapstone 1997). To save space, percent changes > 200% are displayed in 1% incremenents H a v a n n a h I s .…”

“…To allow broad comparison of changes in species abundances among reefs without using multiple significance tests, we only considered a change to have occurred if mean abundances decreased or increased by > 25%. This was based on Thompson & Mapstone (1997), who found little power to detect changes in abundance of < 25% when using the same survey techniques on the same suite of GBR species.…”

Tidal influences on fish distributions on tropical eastern Pacific rocky shores (Colombia)

“…Surprisingly few studies have acknowledged the potential bias that fish censuses carried out at different times of the day or tidal stages may have (but see Kingsford & MacDiarmid 1988, Thompson & Mapstone 2002, Willis et al 2006, McClanahan et al 2007). Small differences in overall fish assemblage structure, as determined by multivariate analyses, can mask large and predictable changes in the densities of individual species, especially where the dataset consists of many species and there are compositional differences among sites (Willis et al 2006).…”

A novel method to obtain accurate length estimates of carnivorous reef fishes from a single video camera

“…Paradoxically, although abundance and size structure are frequently used as indicators of the health of animal populations, accurate and precise length and abundance data are difficult to obtain for mobile organisms like fishes, which occupy a variety of habitats and show distinct behavioral patterns over a range of spatial and temporal scales (Thresher & Gunn, 1986;Harvey et al, 2001a;Thompson & Mapstone, 2002;Irigoyen et al, 2013). As a consequence, the assessment is hampered by the errors and biases inherent to the monitoring methods used, which may reduce the statistical power to detect small but still biologically important changes Harvey et al, 2001b).…”