Bass Biology 101

Prepared by Aaron Zolderdo, PHD student in DR. Steven Cooke’s laboratory at Carleton University (                                   

Big Rideau Lake is home to both largemouth bass and smallmouth bass. Both bass species belong to the sunfish family (Centrarchidae), which is identified as a group of warm water, ray-finned fish native to eastern North America. There are 37 known fish in the Centrarchidae family; others that live in Big Rideau include rock bass, black crappie, bluegill, and pumpkinseed. Being closely related, both largemouth and smallmouth bass share many similarities in their biology and ecology. Indeed, because of the many similarities, both bass species have been collectively referred to as ‘black bass’. However, before we discuss the biology and ecology of Big Rideau’s largemouth and smallmouth bass populations, it is important to be able to identify and differentiate these species from one another. Both largemouth and smallmouth bass look similar but subtle differences that differentiate these species are readily visible. First, as their names suggest, the mouth parts between the two species are quite different. Comparatively, the largemouth bass has a larger gape, with their maxillary bone (or jawline) extending back behind the eye of the fish; the maxillary bone of the smallmouth bass only extends up to the eye, but not further. In addition, largemouth bass are generally larger in size with a more cylindrical body shape; while smallmouth bass have a deeper, flatter, body shape profile. Largemouth bass are predominantly green in colour with a dark, usually distinct, horizontal line of scales extending from the base of the tail across the body dissipating at the eye socket. Smallmouth bass can also be green in colour, but are more commonly a mix of brown and bronze hence why they are often called ‘bronze backs’. Smallmouth bass also do not possess a dark continuous horizontal line of scales extending the length of the body. Instead, smallmouth bass have vertical bars or ‘stripes’ of dark scales, which can become darker and more prominent during the reproductive period. For descriptive pictures of both species see figure 1 below.

Figure 1: Morphological difference between A) Largemouth bass, and B) Smallmouth bass. Key differentiating characteristics include base colouration, distinct markings, and mouth pats (size and shape).

Figure 1: Morphological difference between A) Largemouth bass, and B) Smallmouth bass. Key differentiating characteristics include base colouration, distinct markings, and mouth pats (size and shape).

Habitat and Feeding

Habitat preferences are largely reflected by the physiology and foraging techniques utilized by both black bass species. Largemouth bass have a higher water temperature tolerance, with optimal growing temperatures between 24-30 oC, and an upper lethal limit of 37 oC. Largemouth bass are generally found in shallow bay areas of lake and rivers, which houses complex habitat structure including abundant coarse woody debris, weeds, and rocks – with a combination of these features being most optimal to support high densities. Smallmouth are found in deeper cooler water areas along shorelines, island points, and rocky shoals. Smallmouth bass generally prefer water temperatures between (20-27 oC), with an upper lethal threshold limit of 32-35 oC.  

Both black bass species are opportunistic feeders, in that they will generally consume any prey item that they can capture and swallow. Both species utilize two main techniques for capturing prey including ram-feeding which require the bass to swim over the pray with its mouth open, and suction-feeding which occurs when bass quickly expand their mouth cavity creating a pressure differential between the inside of the mouth and the outside environment. This results in a suction force inward into the mouth of the fish. Aside from similarities in prey capture, both black bass species utilize different foraging techniques that are reflective of the habitat they occupy. Largemouth bass are considered ambush predators, in that they conceal themselves within their habitat (e.g., under a log) and wait for unsuspecting prey items to enter into their ‘strike-zone’. When this occurs, the largemouth bass explodes from its concealed location with rapid speed to engulf the prey item. In contrast, smallmouth bass roam a large territory traveling from feeding site to feeding site preying upon juvenile fish and large macroinvertebrates. The diets of black bass are entirely dependent upon the prey species present, as well as prey density, within the lake community. Within Big Rideau Lake, largemouth bass predominantly feed upon juvenile centrarchids (e.g., bluegill and pumpkinseed sunfish), cyprinids (e.g., fathead minnows), and crayfish, where as smallmouth bass predominantly prey upon only crayfish and cyprinids.

Spawning and Parental Care

Both black bass species employ a similar reproductive life-history strategy, which involves the males providing sole parental care to their developing offspring until the offspring reach a state of independence. This can take 3 – 4 weeks depending on water temperature. Parental care is an essential life-history stage as the offspring are completely reliant upon the parental males for resources and protection. Water temperature is the largest environmental factor influencing the timing of the reproductive period and the development rate of both eggs and offspring. Spawning and reproductive behaviour are motivated by rising water temperatures during late winter and early spring. In Big Rideau Lake, both black bass species begin spawning during the spring months of May and June. Once the lake reaches a consistent 12 degrees Celsius, this will trigger male bass to migrate into the shallow littoral […..need a defining of littoral somehow here] regions of the lake to establish breeding territories and to construct nests in the bottom substrate. Male bass will use their tail and pectoral fins as tools to excavate their nests. Nests are generally saucer shaped depressions in the substrate which will house their developing offspring. Nest sizes range from 0.5 – 2 meters in diameter, but the nest site (breeding territory) can extend to a 10 m2 diameter around the nest. However, male bass may choose to use aquatic vegetation, stumps, and/or boulders in lieu of constructing a distinct nest depending on habitat.

There are differences in preferred spawning habitat between the two species with largemouth bass preferring weedy, stumpy, back bays creating their nests in a water depth between 0.5 – 2 meters deep; whereas smallmouth bass prefer rocky or gravel substrate adjacent to deep water, with nesting depth generally ranging between 0.5 – 3 meters deep. Good nesting areas within Big Rideau Lake for Largemouth bass include the Little Lake Fish Sanctuary area and the adjacent shallow weedy bays. Smallmouth bass nests are quite prominent throughout the entire main lake basin peppered along rocky/gravel shorelines. Based on various environmental factors influencing the lake temperature, the weedy, stumpy, back bays generally warm at a faster rate compared to the main lake basin. As such, largemouth bass tend to enter into the reproductive period prior to smallmouth bass.

Once the lake reaches the optimal temperature for spawning, 15-17 degrees Celsius, female black bass will move into the shallow littoral regions to spawn with desirable males. Research has shown that females are choosy when it comes to mate choice, preferring to spawn with the largest most active males of the population to increase their likelihood of reproductive success and offspring survival. Female smallmouth bass can produce 2,000-30,000 eggs during a single spawning bout depending on body size, with females producing approximately 7,000 eggs per half kg of body mass. Similarly, female largemouth bass can produce 2,000-25,000 eggs during a spawning bout depending on body size, with females producing 2,000-7000 eggs per half kg of body mass. Eggs are small in diameter, (Largemouth bass = 1.6 mm; smallmouth bass = 2.3 mm) and have adhesive properties which allow them to stick to bottom substrate (rocks, weeds, stumps, etc.) and remain isolated within the nest site despite potential disturbances from water current or wind and wave action. After the female deposits her eggs in the nest, she departs the nest site and the male remains behind to provide sole parental care to the developing offspring until they reach independence.

There are four distinct stages of offspring development including egg, egg-sac fry, swim-up fry, and free-swimming fry. The egg hatching rates can differ between both species, with largemouth bass eggs hatching between 2-7 days, whereas smallmouth bass eggs hatch between 4-11 days. Upon hatching, the larval fry termed ‘egg-sac’ fry remain isolated and immobile within the nest absorbing their yolk sac to provide energy for growth and development. This period can range between 3-7 days for both black bass species. Once yolk reserves have been exhausted, juveniles transition to a diet consisting of plankton and zooplankton. At this stage juveniles are termed ‘swim-up’ fry, as they become mobile within the water column directly above their nest. This period can range between 3-7 days for both black bass species as well.  Ample food availability is essential during the swim-up fry stage to ensure sufficient food energy is available to enable optimal growth. Finally, juveniles reach a size (approximately 25 mm in length) where they are able to consume small macroinvertebrates along with larger zooplankton, and begin to travel outside of the nest site in search of food during daylight hours. At this stage, fry have reach the ‘free-swimming’ stage. Shortly following the free-swimming stage, the fry will achieve independence and the parental male will depart the nest site, leaving the young-of-the-year (YOY) fry to fend for themselves. Collectively, the timeline of the offspring development stages can last up to 4 weeks depending upon water temperature and male condition. Active and vigilant parental care is essential during this period, as offspring are at their most vulnerable life-stage, and are completely reliant upon the actions of the parental male for survival.

During the parental care period, males exhibit various parental behaviours that provide resources and protection to their developing offspring. Specifically, males actively guard their nest against brood predators including, but not limited to, bluegill, pumpkinseed, and even other black bass. Research has shown that parental males can swim distances greater than 40 km in a single day chasing predators and patrolling the nest without actively leaving the nest site (i.e., 10 m2 area). Furthermore, parental males also provide a suite of ‘housekeeping’ duties including fanning the nest to maintain a steady flow of fresh oxygenated water through the nest site; fanning also helps to clear away silt and debris that may build up over time. Indeed the parental care period can be very energetically and physiologically demanding. To compound these energetic and physiological demands, parental males cease active foraging and consequently go with little to no nutritional intake for the majority of the care period. As a result, parental males rely almost entirely on endogenous energy reserves (fat etc.) that have been built up from the previous summer/fall growing period. For this reason, younger smaller males generally are unable to provide the same duration and intensity of parental care as compared to larger older individuals. Indeed, parental males have a finite energy budget available to undertake parental care, thus additional challenges during the care period (e.g., angling capture, increased nest predator burden) can result in premature nest abandonment by the males, resulting in reproductive failure. Natural nest abandonment rates in black bass populations can be as high as 40-60% of all nests in a given year. If the natural nest abandonment rate is elevated by additional challenges it can lead to population level impacts in recruitment and year class strength among younger generations.

Growth and Development

Numerous environmental and biological factors can influence growth and development in black bass species, with water temperatures during the growing period, population densities, and prey availability contributing the largest influences. Black bass spawn in early spring in an effort to provide ample growth opportunity for their developing offspring throughout their first growing season. Being top-predators within the lake community, juvenile YOY must grow rapidly to ensure that they remain ahead of any gape limitations imposed by prey species. In addition, YOY must consume as much food as possible to achieve the largest body size possible prior to the winter starvation period. If conditions are ideal to support growth and development, juveniles can reach sizes between 50-100 mm before the end of their first growing season. To support the energy demands for growth and development, juveniles will transition to a fish based diet prior to the end of their first growing season. This enables then to obtain more energy rich food sources essential to sustain the energetic demands imposed by winter conditions. Research has shown that size matters for YOY winter survival, with the largest individuals having the highest likelihood of survival. If successful through winter, black bass can reach lengths between 120-200 mm by year-one of development. Growth is generally rapid throughout the first years of development, with male black bass reaching sexual maturity between the ages of 3 – 5, whereas females reach sexual maturity between the ages of 4 – 6. Although length-at-age data is not known for bass in Big Rideau Lake, this information can be inferred from data collected from surrounding areas. Length-at-age data for smallmouth bass in the Algonquin Park region is approximately 350 mm by age 7. While largemouth bass in more southern regions can reach a length of approximately 400+ mm by age 7. Black bass are relatively longed lived species with the oldest recorded age of largemouth bass being 15 years, and smallmouth bass being 23 years. However, the upper age limit on these species is subject to debate as many factors can influence survival including habitat quality and predation (e.g., angling harvest). Furthermore, current aging techniques require lethal sample for the most accurate measurement of age, creating moral and ethical conflicts among researchers, resource managers, and the public, as killing large ‘healthy’ fish for age determination is not entirely justifiable.

Suggested further readings and references:

Cooke, S., & Philipp, D. P. (Eds.). (2009). Centrarchid fishes: diversity, biology and conservation. John Wiley & Sons.

Philipp, D. P., & Ridgway, M. S. (2002). Black bass: ecology, conservation, and management. In American Fisheries Society, Symposium (Vol. 31). Bethesda Maryland.

Cooke, S. J., Philipp, D. P., Wahl, D. H., & Weatherhead, P. J. (2006). Energetics of parental care in six syntopic centrarchid fishes. Oecologia, 148(2), 235-249.

Fisheries Management Plan for Fisheries Management Zone 18,  February 2016 (accompanied by Background Information document). Ontario Ministry of Natural Resources and Forestry.

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