The question does not have a definitive answer, but there is some evidence to suggest that shad actually attempt to eat a fly or lure, rather than as has oft been suggested—nip at it defensively.

The bright colors and general shape of shad darts resemble krill (above) and copepods (below) which are important shad food sources.

Boyd Kynard, Fisheries Biologist at the Conte Anadromous Fish Research Center in Turners Falls, Massachusetts, believes shad are probably reacting to a feeding instinct.

"When adults enter the river in April and early May, there are few zooplankton. [It] Would not be a good evolutionary strategy to delay migration and feed on these few zooplankton, but some shad could harbor an instinctive response to strike (attempt to feed) on a fast moving object. Based on the present information, a feeding response is most likely the root cause of the dart strike."

As to the premise of defensive or antagonistic behavior, Boyd said "No one has studied the behavioral interactions among individual shad. Do they have antagonistic interactions to determine position in the school? We do not know if they have an antagonistic response toward anything." Much less a shad fly.

John Walter of the Department of Fisheries Science, Virginia Institute of Marine Science, Gloucester Point, Virginia, is a marine biologist and has studied shad feeding in the ocean.

"These questions bring up an unresolved issue of whether feeding during the spawning migration represents 'impulsive' or 'defensive' strikes rather than actual feeding behavior. Anadromous fishes such as shads, salmonids, lampreys have often been lumped into a single life history strategy called anadromy, and behavior patterns of one species have been wrongly attributed to other disparate species. Feeding behavior during the migration is a good example. Pacific salmonids and lampreys become non-trophic and lose the ability to digest food during the migration. These species also die after spawning. Atlantic salmon and shad do not lose the ability to feed and they may or may not die after spawning. The cessation of feeding during the migration is not irreversible and must be due to either a lack of suitable food due to a separation from oceanic food sources, a behavioral change either due to a focus on migrating or spawning or a combination of the two."

But do shad actually recognize a fly or dart as something to eat, say, the same way a trout recognizes a mayfly, caddis, and stonefly? Walter explained the shad marine diet.

"American shad consume some fish but feed mainly on planktonic crustaceans such as copepods, mysid shrimp, and euphausids (krill). Hickory shad (Alosa mediocris) are more piscivorous and routinely consume fish. Very doubtful [that shad think flies are plankton.] The size of shad prey is generally much smaller than the size of the lures commonly used. Copepods are generally less than 1 mm. Mysid shrimp may be as large as 15 mm, slightly smaller than most shad darts. Shad spoons likely resemble small fish. American shad consume some fish but feed mainly on planktonic crustaceans such as copepods, mysid shrimp, and euphausids (krill). Hickory shad (Alosa mediocris) are more piscivorous and routinely consume fish."

When one compares shad flies and darts with the copepods, mysids, and krill, the resemblance—perhaps only incidental—is striking. For example, certain features like round heads, sparse tails, and pink are strongly evocative of the copepod shape and reddish oil seen through their transparent carapace. The two-tone, cone-shaped darts resemble the shape and color of krill and mysid shape, ingested organic matter looks bright green or chartreuse through the carapace. Could the shad's lack of parallax vision and thus inaccurate depth perception be a factor? A small object up close looks large, and thus the shad may not be sensitive to the disparity in size.

Walter considered this. "Perhaps. But maybe actually the size of the shad lures are not that much different than other shad prey such as mysids, krill, and sand shrimp that may reach 5-6 mm in length. So I think the disparity in size between copepods (prey that are probably consumed more in a filtering capacity) and shad darts may be explained by the fact that other prey commonly taken is larger. This larger prey most definitely would have to be attacked as it could attempt to avoid the predator, thus the shad could not simply filter the water unless prey concentrations were extremely high. They are in some areas but probably a shad has to spot and chase down the larger prey as it would a shad dart."

Walter also pointed out that color would make less difference the deeper the fly is fished, but as the photos reveal, light at shallower depths would reveal color through the transparent body of the plankton.

While a remote possibility, the freshwater prey of shad fry might also resonate with adult shad as food targets. Boyd Kynard identified the primary river food source as a plankton called cladoceran bosmina, and they bear a resemblance to copedods but without any tail.

"It is likely that the shapes and perhaps colors of lures do match their natural prey. Shad probably think they are eating some krill-like crustacean." Walter added. "If indeed they do prefer the ones with eyes, perhaps it would be neat to try a shad dart with a single black dot on the top of the jig head as copepods only have one eye. Mysid shrimp and krill have two eyes and are much closer to the size of the shad dart. The shape, color and action of the lure likely mimics some crustacean, ether copepods, mysids or krill. These prey species span a size range from less than a millimeter to larger than 5 cm so they are well within the range of prey naturally consumed by shad."

The observation about eyes is one often noted by shad anglers. Large eyes, or any eyes at all, don't add to the attraction of the fly, and are not a component of the vast array of shad fly and dart patterns. Many contend that an eye actively detracts, and it's been my personal experience that the small black dot suggested by Walter is preferable to none at all.

One of the most integral aspects of shad fishing in rivers is that they seem only to strike a fly at certain places, and almost always where there is some substantial current. I asked John whether turbity and current played a factor in where and how shad feed in the ocean.

"Turbidity likely has an effect upon feeding but various studies conflict upon exactly how. Likely, turbidity reduces the ability to particulate feed and makes filter feeding a more advantageous strategy. As to the relation of prey and currents in the ocean, oceanic currents and ocean circulation determine the distribution of water masses, nutrient availability and ultimately the zooplanktonic food of shad. If you look at a satellite sea surface map you can see great differences in the sea surface temperatures due to oceanic currents. Often it is where temperature breaks occur that large concentrations of oceanic life occurs. It is likely that shad seek these temperature breaks and areas of upwelling due to higher concentrations of prey. Currents also carry along prey in rivers so, yes the association between shad and currents in rivers and in the ocean may be similar."

This leads us directly into the issue of temperature. Anglers know that shad are slow to hit below 54 degrees F. (12 degrees Celsius), and effectively cease to hit once river temperatures exceed 75 degrees F. (24 degrees Celsius). I asked how this corresponds to shad feeding behavior in the ocean.

"I captured feeding American shad in the ocean at bottom temps of 9.4 to 13.2 degrees C. They are most frequently caught in the ocean at between 7 and 13 degrees C. Peaks of upstream migration are at 10 to 15 degrees C. Temperatures are likely higher in the rivers than on oceanic feeding grounds." Shad appear willing to take flies at higher temperatures in the river than they would normally in the ocean.

The deductive reasoning most often applied by anglers as to whether shad are willing to feed in the river is that shad stomachs and digestive tracks are almost always empty, although just last year I had an upriver shad defecate in the net. An obvious counter argument to the empty stomach riddle is that the shad are preoccupied with migrating and spawning and feed infrequently. Another would be that their normal food supply isn't there.

Dr. Jill Leonard, Postdoctoral Researcher at the USDA/ARS Thad Cochran National Warmwater Aquaculture Ctr. in Mississippi has done some stomach sampling of shad along the East Coast, and had some interesting insights into shad feeding behavior.

"I did some work in the Connecticut River taking shad apart and I also found some food in some shad stomachs. I never did much with the data, but I would say generally that they are certainly capable of eating (unlike some salmonids that break down their digestive systems).

"They were generally less likely to have food in the gut the farther upstream that I sampled them (makes sense if it is ocean-based food) and most of what I found was only recognizable as either phytoplankton (green stuff) or zooplankton (brown stuff). I don't really know whether this food was from river feeding or from the ocean, although I would think that some of the fish had been in the river long enough to have emptied their guts of ocean food (that's a guess). Maybe both sources.

I also found occasional other flotsam including fish scales and grass that could have been ingested accidentally or on purpose, but was almost certainly from the river (not ocean). That's all I have for actual data. I will say that some fish I sampled near shore off New Jersey (in the ocean) also had food in the guts and it did not look a lot different from the fish in the river. I never analyzed it for type. Also, I spent a bit of time trying to get migratory adult shad (from the river) to survive in an artificial seawater system and I could get a few of them to start feeding again on brine shrimp."

John Walter noted: "What is interesting and more perplexing is that in the freshwater spawning areas I found an almost complete absence of food in the stomachs. Several explanations may account for this. It may be due to an absence of suitably-sized prey. Larger planktonic prey is far less abundant in freshwater rivers. Since shad will strike at lures during this time and will feed on insect hatches if they occur it may be that the proper food does not exist for shad on the freshwater spawning areas. This fits into the theory of anadromy that one environment is beneficial for juveniles, the river, and another is more beneficial for adults, the ocean. Thus since one environment better serves the needs of different life history stages of the animal, it migrates. The other explanation may be that shad willingly cease to feed on available prey either because their time and efforts are consumed by reproduction or for other reasons such as so as not to eat their young. Other fish cease to feed during spawning so this is not an unlikely explanation. Probably it is a combination of both."

I think the most interesting question of shad feeding during the spawning migration is whether this represents true "feeding" from which shad derive energy or whether it is solely an instinctual response as it is in Pacific salmon. After following fish during the entire course of the spawning migration from the ocean to freshwater I found that shad fed in the coastal ocean, and throughout the brackish estuary. Though the intensity of feeding did not appear as high as in the natural oceanic feeding environment, the feeding appeared directed and of a higher level than would be observed if they were just passively filtering water or instinctually reacting to prey. Unlike salmon, shad retain the ability digest and assimilate food during the anadromous migration and I believe that this limited feeding may serve to mitigate the tremendous energy expenditures experienced during the spawning migration.