DIVE DRY WITH DR. BILL #814: SLAUGHTER IN THE DIVE PARK
Some think that humans are the only ones who kill for "sport." Well, as a biologist I beg to differ. There are other species on Planet Ocean that kill... and not for food or defense. I'm not talking about sea anemones battling one another with their stinging cells for space on the reef. Nor am I thinking of critters that seem to play with their food before chowing down. I'm talking about pure and simple pleasure kills.
One of the worst examples involves two species that I occasionally see right in our dive park, or at least a few miles out to sea from the island. The victim is the ocean sunfish (Mola mola) while the sadistic perpetrator is the California sea lion (Zalophus californianus). Normally I like seeing either of them... at least when the other is not around. However, put the two in close proximity and all Hades can break loose.
I have seen some spectacular massacres in my decades of diving. Dozens of kelp bass attacking jack mackerel trapped inside the kelp; a school of yellowtail chowing down on the same (or any other baitfish that gets in their way!); a knobby sea star engulfing nearly a dozen Kellet whelks; etc. One of the saddest mass killings I've seen underwater involved several sea lions and about a dozen Mola mola right inside our dive park.
The ocean sunfish were all young, some barely over 12" in length. These magnificent fish can reach lengths of nearly 12 ft. and weigh over two tons. Of course like all critters, they have to start out small. They feed on a variety of marine critters with sea jellies being a preferred menu item. However other plankton such as siphonophores and comb jellies and even small fish may be taken. Their relatively small mouths are beak-like and do not close fully.
I remember the first time I saw an ocean sunfish in our waters back in the early 70s. I was leaning over the swimstep on our school launch, retrieving a drifting kelp raft for study in my marine biology lab. As I looked down, I saw what appeared to be the head of a shark heading up toward me. All I could think of was "Jaws" (even though that film would not be released for another three or four years). As it approached the surface, it turned and I saw it was the "swimming head" as some call the Mola.
The incident I alluded to at the beginning occurred in March of 2012. I was diving in the Casino Point dive park and came across a young sunfish that was lying helpless on the bottom. During the course of that dive, I encountered a number of others in varying states of death. Some were still trying to swim while others were lying flat on the bottom. Some were missing their dorsal and anal fins and others were also missing their eyes. However, none of them had their flesh attacked indicating there was no feeding involved.
When I surfaced from that dive, I noted several sea lions in the park. I hadn't seen one underwater on the dive. As I observed them, I could see several had taken up sunfish in their mouths and were playing with them like a frisbie with some of them still alive! Rather disgusting if you ask me. While researching this column I found that sea lions will occasionally take flesh from the softer abdominal region, but most of the fish's skin is too tough for them to bite through.
Given the "frisbie" tossing, it appears the sea lions kill these fish for pleasure. They disable them by chewing off the dorsal and anal fins, rendering them helpless in the water. I'm not sure if they also take the eyes since that could be the work of gulls or subsurface critters. The only other explanation I can think of is that they like Mola mola fin soup. It seems quite similar to the shark fin fishery that cuts off the fins and releases the body, often still alive, to sink to the bottom. However, since sea lions don't cook their food, this is doubtful.
Sadly the International Union for the Conservation of Nature (IUCN) recently listed the ocean sunfish as a "vulnerable" species. The stated reason included declines in some areas of its distribution and frequent bycatch by net fishers, including the swordfish fishery where they composed 29% of the bycatch. If the interaction I've observed with the sea lions is at all common, as suggested by National Geographic, they should be included as an additional threat.
© 2018 Dr. Bill Bushing. For the entire archived set of more than 775 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Dying Mola and anal fin torn off by sea lion; sunfish unable to swim due to loss of fins and dead sunfish on the bottom.
View attachment 522329
Some think that humans are the only ones who kill for "sport." Well, as a biologist I beg to differ. There are other species on Planet Ocean that kill... and not for food or defense. I'm not talking about sea anemones battling one another with their stinging cells for space on the reef. Nor am I thinking of critters that seem to play with their food before chowing down. I'm talking about pure and simple pleasure kills.
One of the worst examples involves two species that I occasionally see right in our dive park, or at least a few miles out to sea from the island. The victim is the ocean sunfish (Mola mola) while the sadistic perpetrator is the California sea lion (Zalophus californianus). Normally I like seeing either of them... at least when the other is not around. However, put the two in close proximity and all Hades can break loose.
I have seen some spectacular massacres in my decades of diving. Dozens of kelp bass attacking jack mackerel trapped inside the kelp; a school of yellowtail chowing down on the same (or any other baitfish that gets in their way!); a knobby sea star engulfing nearly a dozen Kellet whelks; etc. One of the saddest mass killings I've seen underwater involved several sea lions and about a dozen Mola mola right inside our dive park.
The ocean sunfish were all young, some barely over 12" in length. These magnificent fish can reach lengths of nearly 12 ft. and weigh over two tons. Of course like all critters, they have to start out small. They feed on a variety of marine critters with sea jellies being a preferred menu item. However other plankton such as siphonophores and comb jellies and even small fish may be taken. Their relatively small mouths are beak-like and do not close fully.
I remember the first time I saw an ocean sunfish in our waters back in the early 70s. I was leaning over the swimstep on our school launch, retrieving a drifting kelp raft for study in my marine biology lab. As I looked down, I saw what appeared to be the head of a shark heading up toward me. All I could think of was "Jaws" (even though that film would not be released for another three or four years). As it approached the surface, it turned and I saw it was the "swimming head" as some call the Mola.
The incident I alluded to at the beginning occurred in March of 2012. I was diving in the Casino Point dive park and came across a young sunfish that was lying helpless on the bottom. During the course of that dive, I encountered a number of others in varying states of death. Some were still trying to swim while others were lying flat on the bottom. Some were missing their dorsal and anal fins and others were also missing their eyes. However, none of them had their flesh attacked indicating there was no feeding involved.
When I surfaced from that dive, I noted several sea lions in the park. I hadn't seen one underwater on the dive. As I observed them, I could see several had taken up sunfish in their mouths and were playing with them like a frisbie with some of them still alive! Rather disgusting if you ask me. While researching this column I found that sea lions will occasionally take flesh from the softer abdominal region, but most of the fish's skin is too tough for them to bite through.
Given the "frisbie" tossing, it appears the sea lions kill these fish for pleasure. They disable them by chewing off the dorsal and anal fins, rendering them helpless in the water. I'm not sure if they also take the eyes since that could be the work of gulls or subsurface critters. The only other explanation I can think of is that they like Mola mola fin soup. It seems quite similar to the shark fin fishery that cuts off the fins and releases the body, often still alive, to sink to the bottom. However, since sea lions don't cook their food, this is doubtful.
Sadly the International Union for the Conservation of Nature (IUCN) recently listed the ocean sunfish as a "vulnerable" species. The stated reason included declines in some areas of its distribution and frequent bycatch by net fishers, including the swordfish fishery where they composed 29% of the bycatch. If the interaction I've observed with the sea lions is at all common, as suggested by National Geographic, they should be included as an additional threat.
© 2018 Dr. Bill Bushing. For the entire archived set of more than 775 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Dying Mola and anal fin torn off by sea lion; sunfish unable to swim due to loss of fins and dead sunfish on the bottom.
View attachment 522329
