Chapter 16 – Fish

Release Date: April 2013

What does a healthy lake or stream look like? When many of us are asked to describe a healthy lake or stream, a thriving fish community tops the list. Globally, fish are valued for their cultural, economic and ecological importance. In the Credit River Watershed fish are an integral part of the ecosystem and the net present value of the recreational fishery alone is conservatively estimated at $48 million dollars (CVC 2011).

Land use and climate change can have a substantial impact on the types and amount (biomass) of fish living in lakes and streams. Changes in water quantity and quality, stream morphology, riparian vegetation, benthic invertebrates and water temperature also have an influence on the health of fish communities. Fish communities therefore make excellent indicators of stream health. Fish are also easy to identify and are typically present in even the smallest streams and in all but the most polluted water (Karr 1981). This makes fish communities an important integrative indicator of stream health.

While fishing for trout, salmon and bass are popular activities in the Credit River Watershed (Figure 1), most fish in the Credit River and its tributaries are actually members of the minnow family. In fact, over 60% of all fish captured during the 14 years of the Integrated Watershed Monitoring Program (IWMP) are members of the minnow family, with Blacknose Dace (Rhinichthys atratulus) alone making up 20% of all fish captured (Figure 2). Though minnows account for well over half of all fish sampled, they make up just over 30% of total fish biomass due to their small size. On the other hand, the larger fishes of the sucker family account for only 6% of all fish caught during monitoring but make up 35% of total fish biomass (Figure 3), with White Sucker (Catostomus commersonii) alone accounting for nearly a quarter of total fish biomass in the streams.

Figure 1. Brook trout (Salvelinus fontinalis) are a popular target of anglers in the Credit River Watershed.

Figure 2. Blacknose Dace (A) are the most common fish in the Credit River and its tributaries but because of their small size they do not make up most of the fish biomass (i.e. the weight of fish caught). White Sucker (B) on the other hand are not nearly as abundant as Blacknose Dace but because of their larger size account for nearly a quarter of fish biomass in the Credit River and its tributaries.

Fish Monitoring

Fish community monitoring began in 1999 with the establishment of IWMP. Over 14 years (1999-2012) of monitoring, a total of 62 fish species from 17 families have been captured. Fifty-one (82%) of these fish species were found to be native to the Credit River Watershed, whereas the remaining 11 (18%) species are not native to the watershed (e.g. Brown Trout; Salmo trutta is a European species that was stocked and is now naturalized in the Credit River). A total of 110 sites have been sampled as part of IWMP, with 26 sites sampled only once. Seventy-six sites were sampled more than three times and were used in trend analysis.

Figure 3. Percent of the total fish biomass caught during IWMP (1999-2012) by family.

Fish monitoring stations are sampled following Ontario Ministry of Natural Resources (MNR) Ontario Stream Assessment Protocol (Stanfield 2010). Stations are visited once a year, as close to the same time of year as possible. Fish are collected using a sampling method called electrofishing, where electricity is used to temporarily stun the fish. The fish then float to the top of the stream and are collected in nets ( watch a video of electrofishing). All fishes are then sorted by species, counted and weighed. Minimum, maximum and average lengths of fishes are recorded and then all fishes are returned safely to the stream.

Fish community health is summarized using an Index of Biotic Integrity (IBI) score at each site. A fish IBI for the Credit River Watershed has been developed (Morris 1999) representing a range of fish community health values from excellent to poor (Table 1).

Table 1. IBI scores and their associated fish health description.

IBI Score

Fish Health Description

< 5

Poor or very likely impaired

>= 5 < 10

Fair or likely impaired

>= 10 < 15

Good or potentially impaired

>= 15

Excellent or unimpaired

Fish Status (2012)

The current status of fish community health in the Credit River Watershed is based on 60 monitoring stations sampled in 2012 (Figure 4).

Figure 4. Location, status (2012) and trend of fish monitoring stations in the Credit River Watershed.

IBI scores indicate that most monitoring stations are classified as having poor (40%) or fair (30%) fish community health, although nearly a quarter of stations are classified as excellent (Figure 5).

Figure 5. Per cent of fish monitoring stations classified as having poor, fair, good, or excellent community health in 2012.

Most monitoring stations classified as good or excellent for fish health are located in the Upper and Middle Watershed, particularly near the headwaters and in tributaries of the Credit River. Many monitoring stations in the Upper Watershed, however, are classified as having poor fish community health. In some locations, stations with poor fish community health are located close to stations classified as excellent, suggesting that fish communities are responding to local habitat availability and environmental degradation at these stations.

In the Lower Watershed only one station is classified as having good or excellent fish community health, with most stations being classified as having poor fish community health. At stations with low IBI scores, fish communities are composed mainly of fish that are more tolerant of environmental degradation such as Blacknose Dace and Creek Chub (Semotilus atromaculatus).

Fish Trends

Average IBI scores between 1999 and 2012 show the variability in fish community health as you move downstream in the Credit River (Figure 6). Potentially impacted areas downstream of urbanized landscapes or dams are reflected in a general decrease in IBI score moving from Orangeville to Port Credit. The increase in the IBI score near the mouth of the Credit River is not due an improvement in habitat or water quality but rather is the result of increased fish diversity as fish from Lake Ontario enter the mouth of the Credit River.

Figure 6. Average fish Index of Biotic Integrity (IBI) score of monitoring stations in the Credit River and its tributaries. Barriers to fish movement on the main Credit River are shown in red and include 1) Melville Dam, 2) Cataract Falls (natural barrier), 3) Norval Dam, 4) Streetsville Dam.

Overall, average annual IBI scores are showing a statistically significant negative trend across the watershed, suggesting that fish community health is in decline (Figure 7).This overall decline in fish community health appears to be driven primarily by declining IBI scores in the Upper and Lower Watershed whereas IBI scores in the Middle Watershed have remained comparatively stable during the monitoring period (Figure 7).

Figure 7. Trends in average fish Index of Biotic Integrity (IBI) between 1999 and 2012 for the entire watershed (black line) and broken down by Upper (blue), Middle (green) and Lower (red) Watershed. Note the downward trend in fish IBI is statistically significant (p<0.05) for the Upper and Lower Watershed but is not statistically significant (p=0.51) for the Middle Watershed.  

The Upper Watershed is generally thought to be healthier than the Lower watershed, however, there has been a steep decline in IBI in the Upper Watershed. The decline in IBI in the Upper Watershed is primarily driven by a reduction in Brook Trout (Salvelinus fontinalis) and it may be that land use and climate change are having a greater impact on this sensitive coldwater species. The decline in fish community health, particularly in the Upper and Lower Watershed, is of concern and will continue to be closely monitored.


The declining trend in fish community health is cause for concern, particularly in the Upper and Lower Watershed where trends are most pronounced.

Declines in fish community health are likely due to a variety of factors including decreases in water quality from development including water pollution control plants, increased water taking and road run-off resulting in increased storm event flows. Alterations to habitat, removal of stream-side vegetation and climate change are also likely contributing to the decline in fish community health. Because fish are integrators of changes in stream flow, water quality, habitat conditions and climate change, and are also popular from a recreational perspective, they face a suite of challenges for survival.

What is all of this data telling us? Chapter 17 will provide a picture of the overall health of the Credit River Watershed.



CVC (Credit Valley Conservation Authority). 2011. The Value of Recreational Fishing on the Credit River. Ecological Goods and Services Factsheet.

Karr J.R. 1981. Assessment of biotic integrity using fish communities. Fisheries 6: 21-27.

Morris, B. 1999. A Methodology for Assessing the Biological Integrity of Fish Communities of the Credit River Watershed.

Stanfield L. (editor). 2010. Ontario Stream Assessment Protocol. Version 8.0. Fisheries Policy Section. Ontario Ministry of Natural Resources. Peterborough, Ontario. 376 pages.


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