The potential of aerial photography for estimating surface areas of intertidal Pacific oyster beds

Journal of Shellfisheries Research, Dec, 2004 by B.J. Kater, J.M.D.D. Baars

ABSTRACT Pacific oysters were introduced into the Eastern Scheldt in 1964 for breeding purposes. The first spatfall of wild Pacific oysters was recorded in 1976, and a second larval outburst in 1982 definitely settled wild Pacific oysters in the Eastern Scheldt waters. Oyster beds on intertidal and subtidal areas have been growing since. The objective of this study is to research the potential of aerial photography for estimating surface areas of intertidal Pacific oyster beds. Black & white and false-color aerial photographs were used to locate Pacific oyster beds. For verification purposes, oyster bed contours were measured in the field. The accuracy of the method used was comparable with accuracies found in other studies, with a chance of underestimating the surface areas in the field. With aerial photographs of 1980 and 1990 the surface areas of Pacific oysters in both years were reconstructed, showing an increase in surface area of Pacific oyster beds. The study shows aerial photography has the potential to be an aid in surveying intertidal Pacific oyster beds.

KEY WORDS: aerial photography, Crassostrea gigas, reconstruction, Eastern Scheldt

INTRODUCTION

In the Eastern Scheldt Estuary (The Netherlands) flat oysters (Ostrea edulis) have been cultured since 1875 (Shatkin et al. 1997). The estimated stock size in 1962 was 120 million. After the severe winter of 1962/1963, which caused high mortality, the stock was reduced to 4 million oysters (Drinkwaard 1999). Searching for alternatives, Dutch oyster farmers introduced the Pacific oyster to the Eastern Scheldt in 1964. The Pacific oyster Crassostrea gigas (Thurnberg 1793) is native to Japan. The species has been introduced in several other areas, e.g. Australia, New Zealand, France, United Kingdom, Ireland and United States (Shatkin et al. 1997). The reasons for introduction include the economic pressures in the presence of diminishing wild fisheries resources, destruction of a fishery because of disease, or nonexistence of native fishery (Mann 1979).

The introduction of Pacific oysters in the Eastern Scheldt was found acceptable for 2 reasons. The first reason was the expected closure of the Eastern Scheldt with a dam, changing the Eastern Scheldt into a stagnant lake (Nienhuis & Smaal 1994) in which Pacific oysters would not survive. But in 1976 the Dutch government decided to build a storm surge barrier instead of a dam, leaving the Eastern Scheldt a tidal ecosystem (Nienhuis & Smaal 1994). The second reason was that offspring were not to be expected at the Eastern Scheldt latitude (Drinkwaard 1999). However, first spatfall on dike foots and jetties were recorded in 1976, and political pressure stopped importation of Pacific oysters in the following year. A second larval outburst in 1982 definitely settled wild Pacific oysters in the Eastern Scheldt waters (Drinkwaard 1999), and oyster beds on intertidal and subtidal areas have been growing since.

One of the main problems in the Eastern Scheldt concerning expansion of Pacific oysters is the potential interaction with commercially exploited species like cockles (Cerastoderma edule), blue mussels (Mytilus edulis), and cultivated oysters (Crassostrea gigas). Wild Pacific oysters can compete with these commercial species for food and space (Smaal et al. 2001). Although the growth of Pacific oyster distribution has been recognized as a possible problem, surveying oyster beds only started in 1999. To reconstruct former relations between commercial shellfish species and expansion of the Pacific oysters, reconstruction of former surfaces with oyster beds is necessary. Remote sensing techniques like aerial photography can be helpful in this reconstruction. Aerial photography is often used as a tool for surveying e.g. estimating the population size of breeding Greylag Geese (Kristiansen 1997), green algal mats in the Skagerrak (Pihl et al. 1999), vegetation mapping in a tropical freshwater swamp in Australia (Harvey & Hill, 2001), seagrass coverage in Australia (Kendrick et al. 2002), and detection of dead or defoliated spruces in Finland (Haara & Nevalainen 2002).

The first objective of this study is to research the potential of aerial photography for estimating surface areas of intertidal Pacific oyster beds: how well can Pacific oyster beds be seen and distinguished from other structures on aerial photographs? The second objective is to reconstruct the surface areas of intertidal Pacific oysters from the past.

MATERIALS AND METHODS

Oyster Beds on Aerial Photographs

This study was performed on intertidal areas in the Eastern Scheldt (The Netherlands). Figure 1 shows the location of the study area.

[FIGURE 1 OMITTED]

Hardcopies of black & white photographs and false-color slides, made disposable by the National Institute for Coastal and Marine Management/RIKZ, were used in this study. Most photographs were taken on a scale 1:10000, but sometimes a scale of 1:5000 (Verdronken Land van Zuid-Beveland, 1980) or 1:20000 (Roggenplaat 1991 and 1999)was used.