Comment on: “Managing fisheries from space: Google Earth improves estimates of distant fish catches” by Al-Abdulrazzak and Pauly

Abstract

This Comment was prompted by the substantial difference in weir catch estimates in the Gulf between (i) those reported by Al-Abdulrazzak and Pauly (2014. Managing fisheries from space: Google Earth improves estimates of distant fish catches. ICES Journal of Marine Science, 71(3), 450–454), who used Google Earth to count weir numbers, and (ii) those available from official national statistics provided by two major weir fishing countries (Bahrain and Iran). Satellite imageries, including Google Earth, are powerful tools for collecting data on visible structures when verified with adequate ground validation. However, an extension of their contribution to improving overall catch estimates is rather limited without having solid information on daily catch, which will substantially differ according to time and area, and fishing season lengths. It was noted that Al-Abdulrazzak and Pauly (2013) introduced positive biases through their interpretation of Google Earth images and data treatment. They included several assumptions, such as removing the impact of poor visibility, correcting grids of low resolutions, estimating number of unseen weirs, and applying daily catch rates higher than referenced observed values. The overall extent of such potential positive bias could be more than six times that which we considered reasonable. This Comment also corrects misconceptions about “FAO catch data”, discusses other available national data, and introduces the existence of the Regional Commission for Fisheries (RECOFI), a mechanism for fisheries management in the Gulf region, and its recent activities to collect more complete catch and effort data separated by gear.

Ground-truthing the ground-truth: reply to Garibaldi et al.'s comment on “Managing fisheries from space: Google Earth improves estimates of distant fish catches”

“…(4) Both Figure 2a and b of Garibaldi et al (2014), which are supposed to support their assertion [addressed in (3)] that we may have included derelict weirs in our estimate, do not, in fact, illustrate a derelict weir. Rather, Garibaldi et al (2014) have mistaken a maskar, an entirely different gear, for a weir (Arabic: hadrah).…”

“…processes for which both local expertise and restraint in the face of uncertainty are needed. As we will show, lack of local knowledge, in this case, completely invalidated the ground-truthing intended by Garibaldi et al (2014).…”

“…This, it seems to us, is more constructive than entirely ignoring a fishing gear because of data scarcity. We consider it telling that, despite their own local contacts, and access to Governments and national databases in the Gulf region, Garibaldi et al (2014) did not present any alternative estimates.…”

“…In situations such as this, Google Earth-based studies can be invaluable, as they can generate compelling existential and quantitative data on fisheries. Garibaldi et al (2014), while endorsing the use of such studies in principle, criticize the assumptions, methodology, statistics, results, and conclusions of Al-Abdulrazzak and Pauly 2014, primarily based on what they contend is a ground-truthing exercise which, however, only revealed (i) a lack of local knowledge, and (ii) insufficient reflection on the concept and nature of "ground-truthing".…”

“…While this seems reasonable, we point out that these alleged relationships are hypotheses, and that hypotheses are tested with data-which they do not present. Accounting for the relationship between weir catch rates and any of the factors listed by Garibaldi et al (2014; which can be straightforwardly incorporated into the Monte Carlo approach we used) could have improved our results. In fact, we did record the sizes of the three different components of each weir we encountered (Al-Abdulrazzak and Pauly 2014; see Table 2) and, thus, if/when data on catch per weir of different sizes become available, we will be able to consider them, along with the effect of tides and seasons.…”