Bowfin Age & Growth

Bowfin Age and Growth in the Chowan Drainage

The bowfin Amia calva has been called a living relic. It is the sole surviving species from the order Amiiformes, family Amiidae. Native to the Coastal Plain of Virginia, its distribution includes the Blackwater (Chowan) and Nottoway Rivers. Very little is known about wild populations of bowfin in Virginia. Ages up to 9 years have been found in the wild; however, much older individuals have been held in captivity.

In 2007, biologists with the Virginia Department of Game and Inland Fisheries, sought to determine age, growth and sex ratios of the bowfin in the Nottoway and Blackwater Rivers in southeastern Virginia. Total length (mm) and weight (g) information along with sagittal otoliths (sagittae, Figure 1) were collected from Blackwater (N=57) and Nottoway River (N = 60) bowfin. Sex and sexual maturity was also noted for each fish.

Figure 1. Picture of an otolith taken from the bowfin viewed at 10x magnification.

Electrofishing catch rates of bowfin have varied widely over the past several years (Table 1). This led to the discussion of possible causes of such high variability. Water temperature, flow, time of year during which samples were taken were all discussed. It was determined that there was no age data; therefore no determination could be made on year class strength. Filling in the data gaps with bowfin age and growth data was the next step.

Table 1. Electrofishing catch rates of Nottoway and Blackwater River bowfin. Nottoway Blackwater 2007 N = 60 2007 N = 54 2006 – 17 f/hr N = 57 2006 2005 – 44 f/hr 2005 – 31 f/hr N = 90 1996 N = 19 1996 1994 – 3 f/hr N = 2 1994 – 14 f/hr N = 7 1993 – 20 f/hr N = 89 1993 – 23 f/hr N = 76 1992 – 16 f/hr N = 126 1992

Bowfin length-weight relationships followed a tight exponential growth pattern (Figures 2 & 3). All sampling was conducted in late summer (July). No young-of-year fish were seen and few fish less than 400 mm TL were collected. Bowfin grew at similar rates between the two rivers (Figure 4). Fish as old as 9 years were collected on the Blackwater River and as old as 8 years on the Nottoway River. No bowfin younger than 3 years were collected in any sample. Bowfin seem to slow their growth at about 600 mm TL with a maximum total length of around 700 mm.

Blackwater Bowfin

3500

3000 y = 64.838e0.0056x 2 R = 0.965 2500

2000

1500

Weight (g) 1000

500

0 0 100 200 300 400 500 600 700 800 Total Length (mm)

Figure 2. Blackwater River bowfin length-weight relationships

Nottoway Bowfin

4500 4000 y = 80.753e0.0052x R2 = 0.9765 3500

3000 2500

2000

(g) Weight 1500 1000

500

0 0 100 200 300 400 500 600 700 800 Total Length (mm)

2007 BOWFIN Figure 3. Nottoway River bowfin length-weight relationships. 800

700

600

500 Nottoway 400 Blackwater 300

(mm) Length Total 200

100

0 0246810 Age

FFigure 4. Length at age of Nottoway and Blackwater River bowfin.

Year class strength was highest on the Nottoway in 2003. Spawning success (determined by number of individuals collected from each age class) was average or below average for 1998 – 2002 and 2004 (Figure 5). The Blackwater River followed a similar pattern of year class strength. 2003 was a banner year in terms of spawning susuccess, while the all other years were average or below average (Figure 6).

Nottoway Bowfin

30 25

Number 15

10 5

0 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Year

FFigure 5. Nottoway River bowfin year class strength.

Blackwater Bowfin

Number 10

0 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Year

Figure 6. Blackwater River bowfin year class strength.

Sex ratios on the Nottoway River were significantly different from the Blackwater River. Male-female ratios were close to 50-50 in the Nottoway; however, there were more female bowfin than male bowfin collected in the Blackwater (Figure 7). Nottoway female bowfin were significantly larger than females in the Blackwater. OOverall female bowfin were much larger than their male counterparts (Figure 8).

Male 15 Female

Blackwater Nottoway

Figure 7. Sex ratios of bowfin in the Nottoway and Blackwater River

Average Male & Female Bowfin Lengths and Weights

2000 1800

1600

1400 1200 Blackwater Female Bowfins Blackwater Male Bowfins 1000 Nottoway Female Bowfins 800 (g) Weight Nottoway Male Bowfins 600 400

200 0 0 100 200 300 400 500 600 700 Length (mm)

Figure 8. Average lengths and weights of male and female bowfin.

In summary, electrofishing catch rates of bowfin will probably remain very variable. However, age data (from otoliths) is a viable way to collect year class strength information that may help explain that variability. Biologists need to collect more bowfin less than 400 mm TL (or 3 years). Older age classes (> 7 years) also need to be collected in future sampling efforts. Blackwater and Nottoway River bowfin, grow fast (500 mm or 20 inches by age 4) compared to other top predators such as largemouth bass. Female bowfin are on average larger than their male counterparts. Year class strength was similar between the two rivers; 2003 was a very successful year, all other years were average or below.