Developing methods for assessing abundance and distribution of European oysters (Ostrea edulis) using towed video

Abstract

Due to large-scale habitat losses and increasing pressures, benthic habitats in general, and perhaps oyster beds in particular, are commonly in decline and severely threatened on regional and global scales. Appropriate and cost-efficient methods for mapping and monitoring of the distribution, abundance and quality of remaining oyster populations are fundamental for sustainable management and conservation of these habitats and their associated values. Towed video has emerged as a promising method for surveying benthic communities in a both non-destructive and cost-efficient way. Here we examine its use as a tool for quantification and monitoring of oyster populations by (i) analysing how well abundances can be estimated and how living Ostrea edulis individuals can be distinguished from dead ones, (ii) estimating the variability within and among observers as well as the spatial variability at a number of scales, and finally (iii) evaluating the precision of estimated abundances under different scenarios for monitoring. Overall, the results show that the can be used to quantify abundance and occurrence of Ostrea edulis in heterogeneous environments. There was a strong correlation between abundances determined in the field and abundances estimated by video-analyses (r2 = 0.93), even though video analyses underestimated the total abundance of living oysters by 20%. Additionally, the method was largely repeatable within and among observers and revealed no evident bias in identification of living and dead oysters. We also concluded that the spatial variability was an order of magnitude larger than that due to observer errors. Subsequent modelling of precision showed that the total area sampled was the main determinant of precision and provided general method for determining precision. This study provides a thorough validation of the application of towed video on quantitative estimations of live oysters. The results suggest that the method can indeed be very useful for this purpose and we therefor recommend it for future monitoring of oysters and other threatened habitats and species.

Forty questions of importance to the policy and practice of native oyster reef restoration in Europe

“…Other options include acoustic methods, foreshore sampling at low tide and diving campaigns, but the former is unsuitable for many of the metrics that restoration projects need to monitor (such as oyster size and density), while the latter is restricted by both depth and conditions at sea, which limits its utility in many situations. It is therefore critical that novel submarine technologies and monitoring methods are explored; indeed some are already in the process of development (Thorngren, Dunér Holthuis, Lindegarth, & Lindegarth, 2017). A further monitoring challenge is presented by the low concentration of the larvae of this species which makes it difficult to identify its presence in the water column.…”

Assessment of the population of Ostrea edulis in Sweden: A marginal population of significance?

“…Based on the findings of this study, high‐density areas or beds could, however, be selected and studied in more detail (i.e., demographic structure and spatfall), using, for example, scuba diving. Furthermore, previous evaluations of the method have shown that burial and coverage under other oysters or vegetation may lead to underestimation of abundance by on average 20% (Thorngren et al, ). Using common practice for corrections of sampling efficiency (Powell, Ashton‐Alcox, & Kraeuter, ; Tully & Clarke, ) that would mean the estimated size of the Swedish O. edulis population may be raised by a factor of 1.25–45.7 million adult oysters.…”

“…Sampling of oysters was carried out using a method developed and evaluated during spring 2013, described in (Thorngren, Holthuis, Lindegarth, & Lindegarth, ), with the difference that a flat‐bottomed boat with a step‐less electric engine was used to maneuver the sled. Once in position at a sampling site, a random number table (with the accuracy of 0.1 m) was used to determine the specific sampling depth within the strata.…”

“…If this depth was found within 100 m from the original sampling position, two 20‐m video transects were recorded along the depth curve, using a sled with a downward‐facing, high‐definition (HD) GoPro Hero 2 camera (color, 1,080 p) mounted 50 cm from the seafloor, giving a picture frame size of 0.45 × 0.8 m. This gave a total filmed area of 32 m 2 per site, which result in an estimated error of approx. 0.6 oysters/m 2 (Thorngren et al, ). Speed was kept below 0.4 knots to obtain sufficient image quality.…”

“…The videos were imported to iMovie (version 9.0.9), where a number of living and dead O. edulis were estimated by a trained observer as described in Thorngren et al (). The analysis included individuals larger than approximately 4 cm, since identifying smaller individuals has been shown difficult on video (Sallén Lennerthson & Lindegarth, unpublished data).…”