Dear Ocean-Loving Texan,
First of all, congratulations on your long association with the ocean, and thanks for reaching out to us to learn more. We are honored.
This is an interesting observation. We typically hear and think about recent increases, not decreases, in jellyfish populations.
In many places, jellyfish populations are increasing. Commonly, the reasons for this are thought to relate to overfishing and changes to the ocean environment that favor jellyfish. Many of our highly fished commercial species eat jellyfish. When there are less predators around, there are more jellyfish. Also, when we put more nutrients into the oceans, we promote plankton growth, which is the food base for jellyfish. More food also equals more jellyfish.
But, you are noticing jellyfish declines.
The best place to start is with the admission that I don’t know why you are observing this, but I will speculate. In keeping with that admission, it is better if we know more about your observation. For example, is it a single species or several species you are noticing in decline, is it seasonal or year-round, and is this occurring in a specific location or a broad area?
But, from a population biology level, here are a few ideas:
A decreased food base for jellyfish – there is less of the plankton they rely on available. This seems unlikely as we continue to, on a large scale, add more nutrients to the ocean.
Increases in ocean temperature, which are very real, favor different creatures in different areas. Your jellyfish may have moved to cooler areas because your water is now too warm for them. This can apply to fish as well, and is a challenge for some commercial fisheries people along the eastern Altantic states.
The prevailing currents may have changed, but this is usually not the case for most areas.
There are more jellyfish predators in your area. It is hard to imagine that there are more non-human jellyfish predators in your area, but the great thing about the ocean is that we really know very little about it - there is always more research to be done and more to discover. The more we learn, the more we realize we don’t know.
It could be that increased fishing, especially shrimping, is catching more jellyfish, and removing them from your waters.
If your observation is being mostly driven by one species of jellyfish, it could be that the individual species is experiencing massive death rates. A species of sea urchin in the Caribbean did exactly that in the late 1990s and nearly disappeared from the entire Caribbean – it was likely a result of a sea urchin disease – kind of like the Bubonic Plague was to humans; they are now making a comeback.
It could also be that your memory is playing tricks on you. Please don’t be insulted, but we (all humans) don’t have great quantitative memories for long periods of time. To really compare numbers and trends, we need to write down numbers year after year so that we can avoid this little artifact of human memory.
So, this is a long-winded “I don’t know the answer” that hopefully has helped you think about different possibilities that may be behind your ocean observations. Please keep us informed about your ocean knowledge. Thank you for sharing your curiosity with us.
Very best ocean adventures to you.