Bits & Pieces: Paddlefish Anatomy & Walleye Recruitment

Natural History: Third Eye of a Spoonbill

Every North American paddlefish, or spoonbill, has a hole in its head. Frequently unnoticed, but easy to see, it appears as an oval at the base of the bill: a light smudge in a live fish, a gaping aperture in the skull. For an adult paddlefish, having a 12- to 15-inch-long bill, the hole is typically almost a half-inch wide and more than three-quarters of an inch long, slightly larger than a man’s fingernail. It’s centered above and between the eyes and is readily observed on paddlefish taken during the snagging season that begins mid-March in Missouri and ends mid-June in Montana.

This anatomical oddity is usually overlooked by anglers and biologists who focus on the animal’s enormous bill. The paddle of the paddlefish is an impressive flat antenna studded with tens of thousands of sensory cells that detect electrical signals produced by the plankton on which paddlefish feed. The inconspicuous spot at its base is an important artifact of the ancient origins of the paddlefish and represents a century-old mystery.

The opening, called a foramen, is a space between the two parietal bones of the upper skull. It’s covered by thin skin and cartilage that form a window admitting sunlight directly into the braincase of the fish, allowing it to experience daily and seasonal changes in light direction and intensity. Such a structure was common and prominent in dinosaurs of the Jurassic and Cretaceous periods, but not so much in smaller animals of modern times. Foramens, though, can be found in some sharks, frogs, and lizards, but in those species, it’s a tiny, barely noticeable structure. The paddlefish, whose lineage extends from the time of Tyrannosaurus, has a foramen that’s, appropriately, more dinosaur-like—large and quickly recognized.

In 1896, naturalist Harrison Garman performed the first and only detailed description of the paddlefish foramen and what lies beneath it. He found a long vertical stalk containing a nerve. Resembling a long-stemmed plant, its flower pressed close to a distant skylight high above its roots, this lengthy nerve terminated in a slipper-shaped end organ close to the translucent window of the foramen with its faraway base embedded deep within the brain. Garman found that the end organ had a central cavity and pigment, like an eye. Such a structure is referred to as a pineal or third eye. Believed incapable of forming detailed images, it detects and processes light signals, and it influences biological processes. Since Garman’s study, however, the workings of the third eye of the paddlefish have gone unstudied.

In other species, however, the third eye is a dosimeter. Measuring and tracking exposure to sunlight, it enables animals to regulate their body temperature and hormone production, which influence their behavior, like basking and reproduction. The third eye of the paddlefish may operate in ways identical to those of other animals, or it could have different functions reflecting its own archaic origins.

In prehistoric times, the third eye probably sensed overhead predators by movements they made and shadows they cast, enabling ancestors of the modern paddlefish to detect and avoid hungry fish-eaters walking in water or flying overhead. If this is still true today, then anglers onshore or in boats that are facing the sun should have greater success paddlefishing than those with the sun at their backs. Why? Because they would be unseen by the third eye of the spoonbill.

–Jan Jeffrey Hoover

Fishery Science: Mysteries of Walleye Recruitment

Walleyes provide economically and socially important recreational, commercial, and subsistence fisheries. Essential to sustaining these valuable fisheries is good recruitment, the addition of catchable fish to the population. Declining or unstable walleye populations in many waters has attracted attention of fishery biologists for many years, but at times remedies have remained elusive. To better understand walleye recruitment and guide future management, a multi-agency team reviewed all available scientific studies on walleye recruitment published since 1970 and then solicited input of 149 fishery professionals with experience studying and managing walleye populations across North America.*

Walleye recruitment is a complex process that begins with successful spawning, eggs hatching, the young surviving the frail larval stage and then surviving their first summer and winter as small fish vulnerable to predators and starvation, then evading predators for two or three more years to reach adulthood and harvestable size. Survival from egg through larval stages is most affected by abiotic variables, physical and chemical factors like temperature and water clarity. As they grow, biotic variables such as food supply and predation become the primary determinants of survival.

The 108 published studies considered numerous abiotic and biotic variables, and not all variables were considered in all studies. Walleye larvae survival was found to be positively affected by prey availability in most studies but was also positively related growing-season length in two studies. Survival of age-0 walleyes (survival through the first summer) was found to be positively related to spring temperature, prey availability, water level, and growing-season length in four or more studies; but these same variables had either no effect or a negative effect on age-0 walleye abundance in other studies. Spring temperature, water level, prey availability, and yellow perch abundance were positively related to age-1 or juvenile walleye survival. Adult recruitment or growth was positively related to habitat quality, productivity of the water body, water level, lake size, prey availability, and growing-season length in five or more studies. However, growing-season length also had negative effects on growth or recruitment of adults in some studies.

Responses from the 149 experienced walleye experts indicated prey availability, followed by adult walleye abundance, predation, spring warming rates, spawning habitat, lake size, and growing-season length were the top factors affecting walleye recruitment. Yet, some of the respondents noted examples where these same factors had negative or no effects on walleye recruitment. Negative factors affecting walleye recruitment most often reported by most experts were predation and competition.

Walleye recruitment is, fundamentally, a simple process that has evolved over millions of years. But it now occurs in increasingly dynamic environments and, often, in systems with reduced adult (spawner) abundance. These changes not only affect walleyes but also their prey. Complicating these changes is that some of the players in the ecological drama such as yellow perch are prey for later walleye life stages but predators on early life stages, such that changes in their abundance could have positive or negative effects on walleye recruitment. Also apparent from the studies surveyed and the expert opinion is that some factors can both positively and negatively affect walleye recruitment; in other words, these recruitment drivers can be system specific. Lacking a one-size-fits-all management strategy, effective walleye management will largely depend on experienced and innovative fishery managers responding to prevailing conditions.

–Dr. Hal Schramm

*Krabbenhoft, C. A., and 12 co-authors. 2023. Synthesizing professional opinion and published science to build a conceptual model of walleye recruitment. Fisheries 48:141-156.

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