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Gizzard Shad: Do They Deserve Their Bad Reputation? - March 5, 2017

              Each year a natural pageant takes place throughout the fresh waters of the middle and eastern United States. Unlike the awe-inspiring thousand-mile journeys of birds and butterflies, or the life giving salmon runs of the Pacific Northwest, virtually no one looks forward to this particular spectacle of nature.  In early March through April, potbellied silvery fish wriggle by the thousands to spawn in the shallow waters of lakes, rivers, and streams south of the Great Lakes. This lesser spectacle is played out by the gizzard shad, a common fish in the herring family aptly named for its grit filled, food grinding gizzard. As the sun sets on warming spring waters, these weak-jawed, irregularly scaled, toothless fish speckle the submerged vegetation with tiny clumps of sticky eggs and then swim back to the safety of deeper waters.  

            Known well by anglers for their unsightly faces, unpleasant smell, and overabundant production of slime, the gizzard shad has few fans as a sport fish or as a culinary star. Add rumors that these shad eat baby bass and you get an ugly fish with a bad reputation. Yet, the story of the gizzard shad is much less cut and dry; while they are certainly not guilty of eating juvenile bass (gizzards are filter feeders that only eat tiny plankton) their presence can have both positive and negative effects on game fish, and on the ecosystem as a whole.  Gizzard shad play a huge part in healthy ecosystems as a major food source for walleye, bass, and trout; they were even introduced to states as far west as Colorado and Arizona as a food source for stocked ponds and reservoirs. They were considered to be a great forage fish to be included in reservoirs that were stocked with striped bass.  Most avid largemouth bass fishermen know that a good shad-imitating crankbait will almost always get a hit, and many pro anglers contribute their wins to correctly reading the movements of shad schools. But there is a catch; while gizzard shad provide an important food source as juveniles, they become inaccessible to most predators very quickly after reaching their full 10-16 inch size. A number of studies have shown that adult gizzards directly compete with bluegill and sunfish populations, lowering their numbers and decreasing overall bluegill size. Even worse, a study done in 2003 found that high populations of gizzard shad can significantly increase turbidity in the water, reducing the hunting ability of visual predators like largemouth bass.

            Are gizzard shad good for the environment, or do they deserve their ugly reputation? Like most things in life, the answer seems to be “good in moderation”. In a healthy ecosystem where gizzard shad are a native species, these chubby fish play their role as a consumer and accessible prey item without upsetting predator-prey balances. However, the negative effects of gizzard shad increase with an overabundant population. Gizzard shad possess a unique blend of traits that make them excellent competitors in polluted and disturbed habitats; they are tolerant of reduced water quality, they can still feed in turbid waters, and they show preferences for muddy-bottomed, vegetation free habitats. Females lay an average of 12,500-38,000 eggs per season, depending on their age. Some individual fish have been recorded as producing 380,000 eggs in a single spawn. Additionally, gizzards grow fast; as much as an inch a month in some southern populations.  As a result, once a water body becomes enriched with nutrients it provides a ripe opportunity for gizzard shad to quickly overpopulate. High growth rates, explosive reproduction potential, and pollution tolerance create a perfect storm of traits that give the gizzard shad dangerous potential to become an invasive species.

             In the Chesapeake Bay area, gizzards are a nonthreatening native species, but pollution-tolerant species like the gizzard shad have been increasing in number since the late 1980’s. In the tidewater rivers of the Chesapeake, gizzard shad are very abundant, in some cases accounting for 70% of sampled finfish, and act as a possible indicator for the bay’s declining health. However, it is important to remember that these shad are not an underlying cause of water pollution; their increased numbers are merely a symptom. In many cases, healthy populations of prey species encourage game species to return once an ecosystem is restored. Bass, pike, and walleye prefer eating soft gizzard shad to other spiny fish like bluegill and perch, and will preferentially eat young shad when they are in high-density populations.  And, while gizzard shad may increase water turbidity, this behavior can increase ecosystem productivity through nutrient mixing.

             Good, Bad, Ugly. Perhaps the gizzard shad deserves all three monikers. At any rate, the gizzard shad is a reminder of the complexity of natural systems, and of their fragility when one aspect is thrown out of balance.

Patrick Simons is a volunteer with the Maryland Coastal Bays Program. 



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