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The survey design protocols are flexible, but it is critical to consider the target species behavior and survey objectives when generalizing these protocols. Literature review and preliminary or initial studies can be used to incorporate target species behavior into survey design. For example, less than one scallop in 12.5 m2 (0.08 scallops/m2) is below sustainable commercial fishing density23. Thus, by sampling four quadrats per station, the station sample area is linked to detecting scallops at commercial density. Additionally, sea scallops are usually aggregated rather than randomly distributed on the sea floor, influencing how station spacing impacts the precision of density estimates24. Several studies using mean and variance data from initial studies examined precision and determine that 5.6 km was the maximum distance stations should be placed apart5,25,26. The systemic sampling design of the survey was influenced by survey objectives. The boundaries of the SAMS zones change frequently and often after surveys have been conducted21,27. Systemic sampling avoids the serious problem of post-stratification of boundaries for spatial estimates that impacts randomly stratified or optimally allocated survey designs20. Uniform allocation of stations also facilitates detection of new scallop recruitment and mapping sea floor sediments and macroinvertebrate distributions28. The one step where it may not be possible to consider target species behavior and survey objectives is the identification of a survey vessel, which is why the protocol begins with this step. A vessel is essential to at-sea sampling and dictates subsequent steps of the survey design. For our protocols, it was vital to engage the commercial fishing industry to foster transparency in survey methods and confidence in survey results. Using commercial fishing vessels was an impactful way to include industry in our methods and the size and capabilities of the vessels allowed for a large, heavy camera apparatus and for survey stations to be sampled within the needed timeline. Further, vessel owners were responsible for all costs associated with vessel use and were compensated through an allocation of scallop pounds awarded by the National Oceanic and Atmospheric Administration through the Atlantic Scallop Research Set-Aside Program29. Though it is not necessary to engage industry in surveys, the size, capabilities, and costs of available vessels must be considered before developing other aspects of the survey design.
The data collection and processing aspects of the protocols present the greatest advantage, but also a limitation of this method. The use of custom software and databases to quantify data within images comes at a substantial cost. However, the use of these products by the SMAST drop camera survey represents an evolution of a program started in 1999 and is not essential. For example, when the program first started, scallop counts were made with pen and paper and free software is now available to measure within images. Similarly, the current digital still camera was chosen as it was capable of detecting all size classes of scallops and allowed for approximately 200% magnification without loss of image quality (Figure 3), but lower resolution, less expensive cameras used earlier in the survey were able to fully detect scallops of commercial size30. As with the survey design protocols, the type of camera should be linked to the resolution needed to detect the target species and achieve survey goals. Capturing images and recording video at each station provides a significant advantage over traditional survey methods by providing the continuous ability to revisit samples and expand the analysis to taxa or habitat characteristics not initially tracked or enumerated. For example, images with sand dollars and other echinoderms originally noted as present or absent in the SMAST database were revisited to quantify their abundance and biomass through time12. In contrast, samples from more traditional survey methods such as dredges or nets are discarded at-sea and cannot be revisited. However, the advances that allow for massive amounts of images to be taken and stored can result in millions of images being collected with only a small fraction being utilized. This is largely due to time and cost restrictions as humans are needed for data extraction and result in large amounts of unutilized information31. Advances in automated detection of animals and habitat characteristics may help to address this conundrum.
Image based survey methods can provide the necessary data to monitor macroinvertebrates and associated habitat, but supplementing the protocols described here with other methods that collect biological samples is ideal. Without a scallop shell-height meat weight relationship, created from dredge-based sampling, biomass estimates would not be possible. Further, the scallop shell-height meat weight relationship varies with time and location on Georges Bank indicating that consistently updating the equation used to describe this relationship is beneficial32. Combining image and physical sample-based techniques also aids in exploring the biases and assumptions of each method. Measuring shell heights of scallops in drop camera images with calipers quantified a measurement bias associated with the curvature of the camera lens and distance from the image center33. Conversely, paired comparisons between images and dredge tows have helped define what proportion of scallops on the sea floor are actually collected and how the proportion changes with scallop size6.
Underwater imaging has been used in the field of marine ecology for decades17,34. However, decreasing costs of high-resolution cameras and data storage have made the approach more practical than in the past. The methods described in this paper can be generalized and have broad applicability, helping to facilitate the development of more image-based surveys. More specifically, the procedures show how results can be used to produce data to help manage sessile invertebrates (Tables 1-2) and contribute to a broader understanding of the marine environment7,9,10,11,12,13,14,15.