Monitoring Changes In Abundance Research Articles

The annotation approach used for marine imagery impacts the detection of temporal trends in seafloor biota

Image-based surveys of the marine benthos are being increasingly adopted as a monitoring tool for habitats and biota, particularly in mesophotic depths (∼30–150 m) which are technically difficult to survey. Many modern tools for these surveys, such as remotely operated vehicles and autonomous underwater vehicles, can capture thousands of images in a single deployment. Turning this into quantitative data typically involves human annotation which is often time-consuming and costly. Percent cover of organisms is one of the most common metrics for monitoring changes in abundance, which may be attained through visual estimation, digitization, or point-count approaches. However, alternative metrics of abundance such as density (direct counts) or presence-absence, as well as metrics that quantify condition (e.g., bleaching) and size-structure can also provide quantitative information for tracking change. Understanding the statistical power of different approaches is critical to designing effective image-based monitoring programs. Given the differing statistical power and time taken using different approaches, program managers need to decide where to allocate resources. Here we use benthic imagery from two long-term monitoring sites in south-eastern Australia to annotate three example morphospecies (morphologically distinct organisms) using three approaches: point-count (percent cover), full count and presence-absence within imagery. Also, we compare the performance of the point count and full count approaches for monitoring bleaching in one of our morphospecies. We use spatio-temporal models to quantify trends in the empirical data and simulations to quantify the power of these approaches to detect different levels of temporal change using different sampling efforts (either 100 or 200 images). Additionally, we examine the additional insights that size-structure information can provide for two morphospecies. We find that the full count approach provides a higher statistical power to detect change than the other approaches for our example morphospecies, including tracking bleaching status. Size-structure information can provide additional insights such as the occurrence of recruitment or mortality events or growth of individuals. We recommend that monitoring programs using benthic imagery should consider the choice of annotation approach as this is likely to impact observed temporal patterns, particularly when the focus is specific indicator species rather than total biodiversity.

Comparison of methods for estimating density and population trends for low-density Asian bears

Populations of bears in Asia are vulnerable to extinction and effective monitoring is critical to measure and direct conservation efforts. Population abundance (local density) or growth (λ) are the most sensitive metrics to change. We discuss the value in implementing spatially explicit capture-recapture (SCR), the current gold standard for density estimation, and open population SCR (OPSCR) to monitor changes in density over time. We provide guidance for designing studies to provide estimates with sufficient power to detect changes. Because of the wide availability of camera traps and interest in their use, we consider six density estimation methods and their extensions developed for use with camera traps, with specific consideration of assumptions and applications for monitoring Asian bears. We conducted a power analysis to calculate the precision in estimates needed to detect changes in populations with reference to IUCN Red List criteria. We performed a systematic review of empirical studies implementing camera trap abundance estimation methods and considered sample sizes, effort, and model assumptions required to achieve adequate precision for population monitoring. We found SCR and OPSCR, reliant on “marked” individuals, are currently the only methods with enough power to reliably detect even moderate to major (20–80%) declines. Camera trap methods with unmarked individuals rarely achieved precision sufficient to detect even large declines (80–90%), although with some exceptions (e.g., situations with moderate population densities, large number of sampling sites, or inclusion of ancillary local telemetry data. We describe additional estimation options including line transects, direct observations, monitoring age-specific survival and reproductive rates, and hybrid/integrated methodologies that may have potential to work for some Asian bear populations. We conclude monitoring changes in abundance or density is possible for most Asian bear populations but will require collaboration among researchers over broad spatial extents and extensive financial investment to overcome biological and logistical constraints. We strongly encourage practitioners to consider study design and sampling effort required to meet objectives by conducting simulations, power analyses, and assumption checks prior to implementing monitoring efforts, and reporting standardized dispersion measures such as coefficients of variation to allow for assessment of precision. Our guidance is relevant to other low-density and wide-ranging species.

Monitoring changes in abundance of gadoids with varying availability to trawl and acoustic surveys

Gode, 0. R., and Wespestad, V. G. Monitoring changes in abundance of gadoids with varying availability to trawl and acoustic surveys. -ICES J. mar. Sci., 50: 39-51. The use of scientific survey data for assessing abundance of commercially important fish stocks has become a managerial necessity in many regions. Errors and bias in estimates from such surveys may thus have great economic impact. In the north-east Atlantic, abundance of cod has been estimated from bottom trawl surveys and acoustic surveys since 1981. In the Svalbard area, abundance indices derived from acoustic and bottom .awl surveys have differed considerably with discrepancies varying from year to year. At low stock abundance and particularly when the population is comprised of young fish, cod are Gstributed close to bottom and are thus optimally located for a bottom trawl survey, but are poorly recorded by the acoustic technique. In contrast, when abundant year-classes attain an age of 2-3 years and older, they are more pelagically distributed and clearly available for acoustic observation. Dependency on only one of the survey techniques could lead to severely biased stock estimates. A combination of estimates from the two methods is at present hindered by the lack of an absolute estimate of abundance, and by uncertainty in the actual volume sampled due to movement of fish during survey sampling. The vertical profile of the acoustic recordings and size com- position and density of the population are factors which will affect fish availability to the two survey techniques differently. Such variation in availability seems to affect gadoid surveys in general. With a limited amount of additional effort changes in avail- ability and selectivity can be monitored during the survey, and observed changes can be incorporated into analyses to compensate for potential bias due to variation in these parameters. Key words: north-east Atlantic, cod, acoustic surveys, bottom trawling, abundance. Received 14 November 1991 ; accepted 15 July 1992 0. R. Gode: Institute of Marine Research, P.O. Box 1870,5024 Bergen, Norway. V. G. Wespestad: Alaska Fisheries Centre, 7600 Sandpoint NE, Seattle, Washington 981 15, USA.