Do bluegill spawn more than once each year in the Northern U.S.?

[Bruce Condello]

We all know that bluegill will spawn multiple times in the Southern U.S., but will they do the same in the North?

This question was addressed by Dr. Dave Willis of South Dakota State University. Dr. Willis is a Distinguished Professor who joined the Department faculty in 1987. He assumed the role of Department Head in 2008. He is also a member of Big Bluegill.

This commentary was originally posted in the world renowned pond management forum "Ask the Boss". His post was embedded in this thread.

"Spawning periodicity, hatching duration, and peak larval densities were described for bluegill in five lakes of the northern Great Plains in Nebraska and South Dakota, USA from 2004 to 2007. Hatching generally began in early June and duration ranged from 28 to 77 days, indicating protracted spawning. Peak larval density was highly variable among lakes and years and was primarily unimodal, with peaks occurring from late June to July. Peak larval density ranged from 2 to 1,760 larvae/ 100m3. Multimodal peaks in abundance occurred in four instances. Although multiple peaks in larval abundance within years were noted at southern latitudes, there were also many instances of a single peak. Larval density and spawning duration were generally lower than other reported studies of bluegill from southern latitudes although geographic location alone did not consistently explain these patterns.

Now, only crazies should continue past this point. Following is the discussion, where we relate our work to other published literature. Sorry for the technical writing, but I suspect some of you will appreciate this.

Bluegills are typically assumed to be synchronous, colonial spawners that exhibit a protracted spawning season, although a small percentage (4.5 – 7.0%) will nest solitarily (Gross & MacMillan 1981; Neff et al. 2004). Our results generally indicated a single peak of larval abundance in most years and lakes, with a few exceptions. Although there are many examples of multiple peaks in larval bluegill abundance or direct observations of multiple spawning bouts, examples of years with a single peak in larval density are also plentiful at many geographic locations (Table 2). The frequency of sampling in our study (i.e. 7-10 d) is likely adequate to identify potential multiple peaks in abundance. Gross & MacMillan (1981) reported adult male bluegill guarding their brood for 7 d, although elevated turbidity precluded direct observation of bluegill spawning or nesting activity in our study. The use of a 1,000-μm mesh trawl is likely effective at capturing newly hatched bluegill (Isermann et al. 2002). Beard (1982) reported a range of four to eleven spawning bouts in three Wisconsin lakes. Dominey (1981) reported that bluegill breeding synchrony was greater within colonies than among colonies. He noted that even neighboring colonies may cycle out of phase. Consequently, the ability to detect distinct spawning events may be limited as a result of potential asynchronous colony spawning events. In addition, daily age estimates have a margin of error of approximately 7 d. This may lead to a loss of resolution on specific hatching days. Notwithstanding, we contend that potential multiple peaks in hatching would be visible with our sampling method, as is supported by our data in most instances.

Larval abundances in our study impoundments were relatively low when compared to those reported in other studies (Table 2). Thus, a plausible hypothesis is that a longer spawning season at lower latitudes may lead to increased larval densities. Beard (1982) reported that longer spawning seasons (mediated by appropriate water temperatures) resulted in a greater number of individual spawning periods in Wisconsin. We found several instances of larval densities from lower latitudes up to 20-fold higher than our observations but most observations were within two to three times of our estimates (Table 2). In addition, several studies at higher latitudes reported lower densities than we encountered. Generally, bluegill populations at lower latitudes may have the potential to reach relatively high larval densities in some years but may also exhibit lower densities comparable to our study. A suite of factors may interact in complex ways, leading to variable larval densities and subsequent recruitment. As a result, direct comparisons to previous research are challenging. Interacting factors may include abiotic factors such as physical habitat, temperature, and weather (Beard 1982; Pope et al. 1996; Jackson & Noble 2000; Casselman et al. 2002), and biotic factors such as food availability and competition (Partridge & DeVries 1999; Rettig & Mittelbach 2002), predation (Houde 1987; Gray et al. 1998; Santucci & Wahl 2003) and lake productivity (Latta & Merna 1977).
The larval duration (as a surrogate of spawning season) in our study ranged from one to two months. In general, latitude and larval duration were negatively correlated (r = -0.81; Table 2). The expected, extended spawning season was observed in Crane Lake, Indiana, where larval bluegills were collected from early June to early September (Werner 1969). Beard (1982) reported bluegill spawning durations from 31 d to 112 d in three Wisconsin lakes. Chvala (2000), who evaluated the reproductive biology of bluegill in two Nebraska Sandhill lakes, found that while larvae were initially collected in both lakes during June, the spawning season was relatively extended in one lake compared with the other. Newly hatched (i.e., 4–6 mm) larvae were collected at Cozad Lake between June 5 and July 24, while newly hatched larvae were only collected from Pelican Lake between June 25 and July 9. Egg-diameter distributions from bluegill ovaries in both lakes had multiple modes, indicating multiple-spawning (i.e., fractional spawning) capabilities. We did not observe the expected inverse relation between latitude and frequency of bluegill spawning bouts. While three or more spawning bouts were common in more southerly waters, we also found evidence of three spawning bouts in one South Dakota study impoundment. However, even at this more northerly latitude, the influence of latitude could not actually be discerned as we found substantial inter-annual variation in the number of spawning bouts within a water body. Thus, geographic location alone certainly does not explain the frequency of bluegill spawning and this topic certainly warrants further investigation."

[Dave Willis]

The larval trawling is actually done with a pretty simple gear. Picture a metal ring about 18 inches in diameter. Now, picture a conical net attached to the ring. Mesh size can vary, but we are currently using one with a square mesh size of 1-mm per side. That lets some of the algae and zooplankton out so the net does not clog, but captures all but the smallest larval fish. The metal ring has a bridle, we attach a rope, and then we pull behind a boat. We can pull it easily with a 40 hp outboard. There is a bucket at the back end of the conical net so we can remove contents at the end of the tow. We typically tow it just beneath the surface of the water body. We can catch most larval fishes that are under perhaps an inch (maybe more like 4/5 of an inch). Longer than that, they are too strong of swimmers, and can avoid our net.

[Bruce Condello]

One of the things biologists do to research these things is "larval trawling", right Dr. Dave? That sounds like a fascinating job to me. :)

[Dave Willis]

The language summarized here is a "bit" technical. In simpler wording, we found that in some years, bluegills in Nebraska and South Dakota appeared to spawn only once. In other years, we found as many as three spawns by tracking abundance in the larval bluegill over time during the summer. Even when there was only a "single" spawn, we commonly found a few larvae later in the year.