This diagram shows the effects of blast fishing on reef slopes. Fish concentrate at the top of the reef slope so fishermen also concentrate their bomb fishing at the top of the reef slope. The blast causes an avalanche of coral and rubble which covers, smothers or destroys the corals further down the slope. Over 110 kilometers of reef were visually surveyed and over 105 of these had very serious destruction of the coral reefs.
The blasts change the 3 dimensional structure of a reef and the blasted areas no longer function to provide food or shelter to the reef inhabitants. Once the reef structure has been weakened or destroyed by blast fishing, it is much more susceptible to wave action and the reef is unable to maintain it’s role as a protector of coastlines. For example, Kapali, a large uninhabited coral cay off southeast Sabah has been heavily blasted; the reef crest is almost functionally non- existent and the reef slopes are completely destroyed. As a result the sand cay is subject to increased wave action, sediment movement patterns have changed and the island has been rapidly eroded.
Lightly bombed reefs are pockmarked with blast craters but most reefs in northern Sabah have a continuous band of coral rubble instead of a reef crest and upper reef slope. The lower reef slope is a mix of rubble, sand and overturned coral heads. Typically, at the base of the reef slope is a mound of coral boulders, which have been dislodged by a blast and then rolled down the slope in an underwater avalanche. The reef slopes are mostly dead coral, loose sand, rubble or rock but occasionally have overturned clams or coral heads with small patches of living tissue protruding from the rubble.
Blast fishing is particularly destructive because of the direct effect on fish and invertebrates within the blast zone. Not only are the preferred species and sizes killed but the blast also kills all the fish in that area. After one blast fishing explosion, over 2500 dead fish were counted in an area of less than 100 m2. These were mostly damsel fish and small fusiliers but also included juveniles and adults of all reef species.
Occasionally, the remains of very large Porites bommies are found as fist sized lumps of rock which have fractured in an angular way.
Click here -The sound you heard was one blast at 11 nm (23 km or 18 miles).
click here to Compare a blast in a bay (note the multiple echoes)
and click here for a blast further than 23 nm distance.