I wish I were talking about the delicious dessert today, but alas, I am talking about something more alarming. Or fortuitous, I guess it depends on if you are a glass half full or glass half empty person. So, what is Jubilee? It is a weird phenomenon that my volunteer, Sam, told me about. Basically, fish just swim into your hand and then you have dinner for the next week. I’m really not kidding about this, you can walk along the beach and pick up handfuls of shrimp, flounder, crab, etc. To give you some perspective on how many fish you can actually acquire in about an hour, here is my new all-time favorite quote: “At a good jubilee you can quickly fill a washtub with shrimp. You can gig a hundred flounders and fill the back of your pickup truck a foot deep in crabs.” Some say it’s the fish gods, others when stars align.
Okay, so if it isn’t the fish gods or stars, what is going on? Jubilee requires specific conditions that generally make them more likely to occur in the summer and early fall months. During Jubilee conditions water becomes hypoxic and fish become oxygen deprived and move toward the shore to gasp a few more breaths (Do fish take breaths? Gulps?) of oxygenated water. Clearly, I am not a biologist. Essentially, jubilees are fish kills, but the fish haven’t died yet, so people get to go collect them by hand.
How does Jubilee occur?
Generally, Jubilee occur when bottom water becomes oxygen depleted (hypoxic) and benthic fish try to escape the poor oxygen conditions by coming to shallower surface waters. There are several factors that influence the water column to make jubilee conditions possible.
- Jubilees occur because of water stratification. Stratification can occur for many reasons including temperature, salinity, and riverine input. For fish, stratification isn’t so great. That’s because strongly stratified waters are difficult to overturn (or mix). No mixing means no distribution of oxygen and nutrients throughout the water column.
- Temperature can influence the stratification of the water column because cold water sinks and warm water rises. It’s the same concept we are taught with air. Warm air rises, warm air holds more moisture, once it rises high enough in the atmosphere it cools, and rains. So, the difference in water is: the sun warms the surface waters which allows them to remain on the surface because warm water is more buoyant than cold water. Occasionally something disturbs the system and you can overturn the water causing mixing (fish like mixing, remember?). Unlike the atmosphere, warm water can’t hold as much oxygen as cooler waters (I’ll come back to this because bottom waters can also become hypoxic). During summer months, in shallow estuaries, the sun can heat the entire water column which causes stagnation (the entire water column is the same density) and decreased oxygen conditions. Stagnation combined with oxygen concentration can create perfect conditions for jubilee.
- Salinity and riverine input go hand in hand. These 2 go together because salty water is heavier and tends to sink, while less saline water floats. Rivers input freshwater into saltier estuarine systems, so the river input floats on top of the estuarine water (this is called a salt wedge estuary). However, not all estuaries are salt wedge estuaries because of things like temperature and flow conditions, but that might be another article in itself. My point is rivers can create stratification just based on salinity differences. Andddd… if I haven’t drilled this point home yet: fish don’t like stratification.
- Remember when I said I would come back to bottom waters being hypoxic? That would be thanks to phytoplankton. When excess nutrients get washed into the estuary (eutrophication), phytoplankton photosynthesize in a feeding frenzy (say that 3 times fast) and multiply rapidly. It sounds like a good thing because photosynthesis produces oxygen, but in reality, once the nutrients are used up, the phytoplankton die and sink. When they sink they are then broken down by bacteria, which uses the oxygen and the bottom water becomes hypoxic. This is a problem for benthic fish, which means fish kills or fish trying to escape the hypoxic conditions leading to jubilee.
- Incoming tides can push Jubilee toward the shore. Once the bottom water is hypoxic, an incoming tide is able to push that water farther into the estuary kind of like a storm front. Pelagic fish (fish that swim in the water column) are able to swim in the top waters that are better oxygenated, but the benthic fish and crabs tend to move toward the shore where the water is shallow and still oxygenated. In the shallows, they’re able to stay on the bottom, while also benefiting from the oxygenated top waters.
- Wind can also influence the water column by creating surface currents that move the surface water layer. As a result, the deoxygenated bottom waters flow in the opposite direction. So, if the wind is blowing from onshore to offshore, bottom waters will start moving toward the shore. This coupled with an incoming tide can be really bad for benthic fish.
By now, it should be apparent that there is a specific set of conditions that allow jubilee to occur, but these factors can fluctuate which varies the duration and strength of a jubilee. The recipe for an especially bountiful jubilee means you need warm stagnant water, eutrophication resulting in a phytoplankton bloom, and winds pushing the surface waters offshore so the incoming tide can push hypoxic water farther into the estuary.
So, when it comes to jubilee, are you a glass half full (free seafood for days) or half empty (fish kills) kind of person?