Black Water: Marlin

Anyone who has spent any time around Honokohau Harbor has seen a few of these strung up at the fuel pier. Marlin get huge, so it is excusable if you gawked in awe at the 400 pound dead sea monster on display, but the lifeless form is missing what makes them really special.  By nearly every measure, living marlin are probably the most impressive fish swimming in our oceans today.  For starters, they are world travelers.  One animal was tagged off the coast of Delaware and later recovered near Mauritius- a journey of over 9000 miles!  At 50 miles per hour, marlin are not only faster than anything most divers have seen underwater, they are faster than most speed boats.  And while that 400 pound animal may be more than twice your bodyweight, at 1400 pounds, the largest animal on record was more than 3 times a big as that.  Thus, I’d like to use this blackwater blog to celebrate the marlin for the oceanic apex predator that it is.  And I’d like to start with a little known species of odontocete called the dwarf sperm whale (Kogia sima).
Dwarf sperm whales have never been photographed underwater.  They are notoriously difficult to approach and are known for slipping under the water and descending just out of sight of humans.  So imagine my excitement as I found myself in the water with my camera as a pod slowly swam toward me!  The boat crew watched in near silence as their fins dipped below the surface, and the animals descended.  I may have seen a shadow in the distance, but that was probably my imagination.  I just floated hoping the animals would get curious and come back to play.  A minute or so later, the boat crew saw me nearly jump out of the water, muffling some sort of alarm through my snorkel.  Angry reef fish are quickly calmed by simply swimming away.  Even aggressive sharks can be stiff-armed.  But what can you do when a 12-foot long marlin charges you in blue water?
Marlin series (1)
Three photos taken in sequence during the encounter.
We have never seen an adult marlin on a blackwater dive, but we have seen quite a few of their young.  Marlin breed in the late summer and into the early fall and are capable of spawning as many as 7 million eggs at one go.  The resulting young (pictured below) will take 2 years to reach sexual maturity but grow at a rate of more than half an inch per day.  Out of that 7 million eggs, maybe one or two will reach sexual maturity and far fewer will reach the legendary benchmark of “grander” (over 1000 pounds).
Istiophoridae larva best 2 watermarked
Istiophoridae larva head on watermarked
Post flexion marlin photographed on a blackwater dive in May of this year (2015)
Fortunately for me, the apex predator granted me life as it decided to turn at the last minute (as seen in the sequence of photos above), but I’m afraid we have not been so sympathetic to the fish.  Blue marlin have declined by more than 30% of their population in just the last 14 years alone and fishing pressure for this species is increasing.  No blue marlin fisheries are considered to be sustainably managed. Thus, like other banner-species of ocean conservation such as oceanic sharks and even bluefin tuna, marlin are in a lot of trouble.  Sure, they provide a lot of meat, but because they are apex predators, pollutants such as mercury bioaccumulate in their tissues, so they have some of the highest levels of mercury (>.5 parts per trillion) of any marine fish!
-Jeffrey Milisen
MSc.
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