Gizzard shad, as adults, mostly forage shallow, muddy lake bottoms. The reason they are called "gizzard shad" is because they have a gizzard. They can break up small mussels, tiny crustaceans and bottom-dwelling insects and worms. They have a subterminal mouth, which is a clue how they behave. Threadfin shad are filter feeders. When they run out of food, they starve. If gizzard shad can access shallow muddy areas, they thrive.
There are many differences between those two species. Although they look similar, they are completely different. Here are a few differences. Understanding those differences help up understand the roles they play in our lakes and ponds.
Threadfin shad:
1) Grow to 7", I've handled millions of them (literally) and have not seen one larger than that.
2) Average lifespan is about 18 months.
3) Because their lifespan is so short, they spawn often, every 45-90 days when the temperature is right.
4) They lay their eggs on grassy substrate, along the shore, starting about an hour before daylight, until shortly after the sun comes up.
5) Threadfins have a huge spawning run in spring, subsequent spawns are made of smaller runs.
6) They are filter feeders, preferring more pelagic waters.
7) Threadfins die when water temperatures drop into the low 40's.
8) Since they prefer mostly open water, and are fairly small, their biggest predator in managed ponds are intermediate-sized largemouth bass. Big bass tend to not pursue threadfins.
Gizzard shad:
1) Grow beyond 17" and can weigh more than two pounds.
2) Average lifespan is 4 years, can live up to 5 or 6.
3) They spawn once yearly, dumping upwards of 250,000 eggs at once into the water column.
4) Young gizzard shad, like many fish, glean their food from the water column, preferring plankton and tiny insects.
5) Grow really fast. I've seen gizzard shad go from egg to 10" in one growing season.
6) I've seen many muddy ponds and lakes where gizzard shad are the cause.
7) With huge numbers and fast growth rates, they can become the dominant species in a pond, if there aren't enough top end predators.
8) This next is an opinion, based on sparse research and many years experience. Gizzard shad, once they reach a certain density, inhibit reproduction, not only of their species, but limit recruitment of other species in that system. I believe (heavy on the word "believe") they give off a pheromone that disrupts spawning. When that happens, nature often makes a correction via a winterkill of aging adult gizzard shad.
General knowledge about each species.
1) When my company manages a lake for trophy bass, we always stock threadfin shad to boost growth of young bass.
2) Once we can estimate that 20-25% of our bass population has reach 2.5 pounds, we'll stock gizzard shad. We don't want gizzard shad to become the dominant species, so we want enough bass large enough to prey on fast growing gizzard shad. Big bass prefer big meals, and gizzard shad provide that.
Those are my shad thoughts for today.
