Change in growth and overall condition in populations of anadromous burbot (Lota lota) in the Gulf of Bothnia

Change in growth and overall condition in

populations of anadromous burbot (Lota lota) in

the Gulf of Bothnia

Mattias Alftberg Melin

Student

Degree Thesis in Biology 15 ECTS Bachelor’s Level

Report passed: 2019-08-30 Supervisor: Pär Byström

Abstract

Many populations of burbot (Lota lota) around the world have been extirpated, are

endangered or are in serious decline both regarding numbers but also in size. The aim of this study was to investigate if growth and overall condition in populations of anadromous burbot in the Gulf of Bothnia has changed over time and if so, discuss potential causes behind. This was done by comparing size at age and individual level condition indices of the two

populations of anadromous burbot in Sävarån and Rickleån to previous studies from the same rivers. The results showed that growth of young burbot has increased between the time period 2001-2014 to 2019 in Sävarån and also a change towards a higher frequency of young individuals and a lack of older ones. Furthermore, an increase over time in condition was observed in Sävarån. In Rickleån the growth at the age of 3 had increased from both 1969-1971 and 2001-2014 to 2019. At the age of 4 to 9 a decrease in growth was shown from the time period 1969-1971 to 2001-2014 in Rickleån. A shift towards warmer water temperatures due to climate change might be an explanation in the observed change in both growth and condition in the population of burbots in Sävarån and Rickleån. Furthermore, the observed change in age frequency in Sävarån could also be a result of an increase in water temperature but could also be an effect of restoration and the control of pH in Sävarån.

Table of contents

1 Introduction ...

1

1.1 Introduction ... 1

1.2 Aim of study ... 1

2 Method ...

2

2.1 Site descriptions ... 2

2

2

2.2 Sampling ... 2

2

3

2.3 Laboratory analysis and data processing ... 4

2.3.1 Historical data on burbot ...

4

2.3.2 Age ...

4

2.3.3 Size and Growth ...

5

2.3.4 Organs and condition ...

5

2.4 Statistics ... 6

3 Results ...

6

3.1 Age distribution and growth ... 6

3.2 Size and condition ... 8

3.3 Liver-index ... 9

10

4 Discussion ...

10

4.1 Sävarån ... 10

4.2 Rickleån ... 11

4.3 Conclusions ... 12

5 Acknowledgements ...

12

6 References ...

13

1

1 Introduction

1.1 Introduction

In the Baltic Sea region, the surface air temperatures have overall shown a significant increase over the past 140 years (HELCOM 2013). The daily temperature cycle is also

changing and there has been an increase in temperature extremes. Since 1985 an increase in water temperature have been observed in the Baltic sea, the annual mean water temperature has been estimated to have increased by up to 1 C° per decade from 1990 to 2008, and the summer sea-surface in northern parts is estimated to increase by 4 C° by the end of this century. The late spring maximum in river runoff could shift earlier, possibly even into February or January due to the increasing temperatures as well as from changes in the annual cycle of precipitation and increased evaporation (HELCOM 2013).

Since nearly all aquatic organisms are poikilothermic, the increase in water temperature due to global warming will affect whole aquatic ecosystems to a large extent (Huss et al 2019). Changes in ambient water temperature has the largest effect of all abiotic factors on physiological properties in fish. It will affect metabolic demands, foraging, digestion rates and assimilation efficiencies (Persson et al. 1998, Byström et al. 2006, Huss et al. 2019). This is supported by Hofmann and Fischer (2002) that states that food supply and temperature are the two most important variables determining growth of fishes.

The burbot (Lota lota) is a member of the cod family and as its marine ancestors the species prefers cold waters (Stapanian et al 2010). The population of burbot in the Baltic Sea is anadromous; it dwells in the sea, but spawning occurs upstream of rivers (Hudd and Kjellman 2002). Burbots spawn during winter between January and Mars at temperatures ranging from 0 C° to 6 C°. After spawning, adult burbots descends to the Baltic Sea after not more than a couple of weeks with some few individuals as late as May-June (Müller 1987). Older and larger burbots tend to prefer lower water temperatures than juveniles; between (Hofmann and Fischer 2003) and Kjellman and Eloranta (2002) have also suggested that growth of larger burbots is less temperature depended than for small individuals. Müller (1987) further have shown that adults spend the warmest summer months in cold deep-water habitats with low resource abundance. This may lead to a decline in all-year-growth in older burbot. On the other hand, in the northern Baltic Sea, i.e. the Gulf of Bothnia, the sea will heat up and cool down slowly since it is a large water body, which will enable burbots to use the shallow bays with high resource abundance for a longer time period during the summer which may increase all-year growth of burbots in the Gulf of Bothnia (Sandberg 2015). Many populations of burbot around the world today have been extirpated, are endangered or are in serious decline both regarding numbers but also in body size. Due in part to the lack of commercial and sportfishing interest in burbot, burbots are rarely consider burbot in

management plans (Stapanian et al. 2010). The burbot is listed as near threatened (NT) in the red list of Sweden (ArtDatabanken 2015). One reason to this appears to be habitat change, especially the effects of dams (Stapanian et al. 2010). Another element seems to be episodic acid runoff as acidification may cause recruitment failures in burbots (Hudd and Kjellman 2002). Moreover, increases in water temperatures due to climate change have been suggested to decrease survival of larval burbot due to increased cannibalism (Barron et al. 2012). Increase water temperature may also force large adult burbots to migrate out to cooler and low productive deep habitats with reduced growth as a consequence (e.g. Muller 1987).

1.2 Aim of study

The aim of this study is to examine if growth and overall condition in populations of anadromous burbot in the Gulf of Bothnia has changed over time. This was done by comparing size at age and individual level condition indices of the two populations of anadromous burbot in Sävarån and Rickleån to previous studies from the same rivers. The

comparison will primarily focus on whether there has been a change in growth or not, and this was studied by comparing length at age in the two populations over time. In addition to this, changes in condition of burbot in the two populations were also studied, through tests of changes in condition-index, liver-index, and gonad-index over time. Hence, the main

questions asked in this study were:

• Has there been a change in growth and/or health of anadromous burbots in Sävarån and Rickleån?

• If a change has occurred, what may be the cause behind?

2 Method

2.1 Site descriptions

2.1.1 Sävarån

Sävarån water catchment area is 1161 km2 _{and it is 103 km long.The source lake is}

Lossmenträsket. The bedrock is dominated by gneiss; the main soils are peat and moraine. Wetlands constitutes 22%, forest 68% and lakes 6.3% of the water catchment area. Sävarån is a Natura 2000 area and of national interest. The population of salmon is of particular

interest due to the increase in reproduction during the last decade. In 2017 the number of salmon juveniles was measured to 22 individuals/100 m2_{. Sävarån is affected by two smaller} hydroelectrical power plants, but there is no regulation of the water flow. In 1998 a fish ladder was constructed at one of the hydroelectrical power plants. The river has been heavily used for log driving and in 2011 an extensive restoration of the whole catchment began. In the 1980’s Sävarån was heavily acidified, reaching pH-values below 5.0. So, during 1991 a liming program started in the main river, and in 1995 a liming metering distributor were installed. Therefore, the pH-levels during todays’ spring is normally above 6.3 (Västerbotten County Administrative Board 2018a).

2.1.2 Rickleån

Rickleån water catchment area is 1649 km2_{, and lakes represent close to 9% of the area. The} most common component of the bedrock is gneiss. In the upper parts some granite and gabbro can be found. The upper parts are dominated by moraine and wetlands, but in the lower parts of Rickleån glacial sediments, bare bedrock, and lake- and sea sediments are more common. The lower parts of Rickleån, downstream of Robertsfors, is of national interest and it has a valuable population of sea trout. Rickleån is affected by four

hydroelectrical power plants, and all except one are situated in Robertsfors. From 1910 to 2002 the fish could not migrate past Robertsfors, but in 2017 fifteen salmons migrated past the upper hydroelectrical power plant. This change was due to fish ladders which were constructed in 2002, and they were later improved in 2006.The reaches downstream of Robertsfors has never been used for log driving. Rickleån is episodically acidified during the spring, as it reaches pH-values below 6.0 and is not limed (Västerbotten County

Administrative Board 2018b).

2.2 Sampling

2.2.1 Sampling methods

Burbot catch samples were collected during weeks 2-10 of 2019. A total of 74 burbots were caught during this time, and of those were 60 burbots caught in Sävarån and 14 burbots in Rickleån. 26 days of sampling were conducted; 5 in Sävarån and 21 in Rickleån. The sampling was done with regular ice fishing gear and it was carried out mainly between 17:00-21:00 o’clock. The reason behind the chosen method was to replicate the previous study of Sandberg (2015) who carried out sampling mainly between 18:00-22:00, but also since Müller (1973) suggests activity peaks between 18:00-20:00. A jigg-head of 35g and hook size

3

4/0 tackled with 3-4cm fillets of herring were used as bait. The fishing technique used was to bounce the jigg-head at the bottom 4-5 times followed by lifting the jigg-head slightly above the bottom and then holding it still for 5-10 seconds before repeating. This is a well-known fishing technique which often is used when ice fishing for burbot. All fish caught was immediately frozen and stored until laboratory analysis.

2.2.2 Locations

The sampling was performed at five different sites; two sites at Sävarån (figure 1) and three sites at Rickleån (figure 2). All five sites are well known burbot “fishing spots” and are located downstream of the first hydroelectrical power plant. The two sites at Sävarån are located near the sewage treatment plant in Sävar with a depth of around 1m. The three sites at Rickleån are located downstream highway E4 with a depth of 1-3m.

Figure 1. Water depth at the two fishing spots at the river Sävarån. The mean depth of a 10m radius around fishing spot 1 at Sävarån was 0.99m and 1.05m at fishing spot 2. Sources are SLU (2019) and Genesis maps (2019).

Figure 2. The location of the three fishing spots at Rickleån. Source are SLU (2019).

2.3 Laboratory analysis and data processing

2.3.1 Historical data on burbot

Data on past burbot growth and condition variables was obtained from Bengtsson (1973) and Sandberg (2015). The data available in Bengtsson (1973) was collected between 1969-1971 and consisted of sample size, mean values and standard deviation of length at age of burbot in Rickleån. In addition to this, the age distribution with number of individuals per year class was available. The sampling was done with ledger tackle at similar locations as in this study. Sandberg (2015) collected data from both Rickleån and Sävarån between the 2001 to 2014 with the exception of the years 2010-2012. The sampling was done with regular ice fishing gear downstream of the first hydroelectrical power plant as done in this study. The data available were sample size, mean value and standard deviation of length at age for burbots. The age distribution of the burbot in both rivers was calculated from Sandberg’s (2015). A mean gonad-index for males and females, a mean liver-index both per sex and combined, and a mean condition-index together with sample size and standard deviation were also available. 2.3.2 Age

The age of each individual was determined by otolith reading. The otoliths pairs from each captured individual were obtained thorough dissection in the lab. Thereafter each pair was cleaned and dried before stored in marked envelopes. Annual growth rings on each pair of

5

otoliths were counted using a dissecting microscope set between 6.9-40x magnification (figure 3). The otoliths were put in 70% ethanol before reading in order to enhance visibility. Also, the reading was performed in different directions and angels, and comparisons were made between pairs to increase accuracy.

Figure 3. Otoliths viewed in dissecting microscope. Female estimated age 5 years, 51.5cm and 775g from Sävarån.

2.3.3 Size and Growth

The total length to nearest millimeter of all individuals was measured from the tip of the snout to the end of the tail. The weight was measured using a sartorius LP6200s master pro scale. A <size comparison was made for each river by comparing the burbots’ total length (in millimeter), and total weight (in grams) from the three different time frames. Growth was defined as the increase in length at age.

2.3.4 Organs and condition

The weight of gonads and livers was measured using a sartorius LP6200s master pro scale. Indexes for condition, gonads, and liver where then calculated using the Fulton’s formula: 𝐼 = 10&_{𝑊/ 𝐿}*

𝐼 = 𝐶𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 𝐼𝑛𝑑𝑒𝑥

𝑥 = 𝐶ℎ𝑜𝑠𝑒𝑛 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑡ℎ𝑎𝑡 𝑏𝑟𝑖𝑛𝑔 𝐼 𝑐𝑙𝑜𝑠𝑒 𝑡𝑜 𝑢𝑛𝑖𝑡𝑦, 𝑖𝑛 𝑡ℎ𝑖𝑠 𝑐𝑎𝑠𝑒 5 𝑜𝑟 6 𝑊 = 𝑊𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑔𝑟𝑎𝑚𝑠, 𝑊_A 𝑓𝑜𝑟 𝑙𝑖𝑣𝑒𝑟, 𝑊_D 𝑓𝑜𝑟 𝑔𝑜𝑛𝑎𝑑 𝑎𝑛𝑑 𝑊_E 𝑓𝑜𝑟 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 𝐿 = 𝑇𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔ℎ𝑡 𝑖𝑛 𝑚𝑚

2.4 Statistics

Since raw data was not available from the previous studies, assumptions that the

requirements for doing t-tests are met in the original data were made. These assumptions were strongly motivated as the authors stated that data were normally distributed (Bengtsson 1973. Sandberg 2015). Mean values, standard deviation, and sample size could in all be obtained from the previous studies. For visual clearance in figures, a 95% confidence interval was calculated for all data and presented as error bars in result figures. Independent samples t-test was used to test significance. This was done by calculating a t-value using the formula:

𝑡 = 𝑋HI− 𝑋HK L(𝑆IK

𝑛I+

𝑆_KK

𝑛K)

The p-value was calculated in Microsoft excel with the formula “=T.DIST.2T(t-value, degrees of freedom)”.

3 Results

3.1 Age distribution and growth

The age distribution in Rickleån differed between the different years of samplings (figure 4). The age distribution in 1969-1971 was centered around the age 5 to 6. In 2001-2014 the age was centered around 6 to 7 years and in 2019 the age was centered around 3 to 4 years. However, note that the lack of older burbots in 2019 could be due to the low sample size of 14 burbots.

Figure 4. Age distribution in Rickleån between 1969-1971, 2001-2014 and 2019. The sample size in 1969-1971 was 244 individuals. The sample size in 2001-2014 was 216 individuals. The sample size in 2019 was 14 individuals.

The age distribution in Sävarån differed between 2014 and 2019 (figure 5). In 2001-2014 the age was centered around the years 6-7 and in 2019 it was centered around 3-4 years old with nearly half of the individuals being of an age of 3 and only one burbot older than 6 years was caught in 2019.

0,00% 5,00% 10,00% 15,00% 20,00% 25,00% 30,00% 35,00% 2 3 4 5 6 7 8 9 10 11 12

F

re

que

n

cy

o

f i

n

d

iv

id

ua

ls

(%

)

Age (years)

Age distribution - Rickleån

1969-1971 2001-2014 2019

7

Figure 5. Age distribution in Sävarån between 2001-2014 and 2019. The sample size in 2001-2014 was 300 individuals. The sample size in 2019 was 60 individuals.

A change in length at age in burbot from 1969 to 2019 in Rickleån (Figure 6). Burbot of age 3 years was significant longer in 2019 than in both 1969-1971 and 2001-2014 (p<0.01),

indicating a higher growth rate in early years in 2019. At the age of 4 to 6 years, burbots in 1969-1971 were significant longer (p<0.01) than in 2001-2014 but no difference could be found when comparing the former year with 2019. For age 7 to 9 the mean length of burbots in Rickleån were significant higher in 1969-1971 compared to 2001-2014 (p<0.01). This indicates a decline in growth from 1969-1971 to 2001-2014 in burbots in Rickleån.

0,00% 5,00% 10,00% 15,00% 20,00% 25,00% 30,00% 35,00% 40,00% 45,00% 50,00% 2 3 4 5 6 7 8 9 10

Fr

eq

ue

nc

y

of

in

div

id

ua

ls

(

%

)

Age (years)

Age distribution - Sävarån

2001-2014

2019

L

en

gt

h

(m

m

)

Age (years)

Length at age - Rickleån

1969-1971 2001-2014 2019 P < 0.01 P < 0.01P < 0.01 P < 0.01 P < 0.01 P < 0.01 P < 0.01 (0, 0, 1) (40, 13, 4) (68, 28, 3) (73, 54, 2) (23, 54, 0)(11, 44, 0) (9, 15, 0) (8, 2, 0) (1, 2, 0) (0, 2, 0) (14, 3, 4)

Figure 6. Length at age of Rickleån between the time periods 1969-1971, 2001-2014 and 2019. Sample size in brackets.

An increase in mean length in burbots in Sävarån was present for the age 3 to 6 years old from 2001-2014 to 2019 (P<0.01) (Figure 7). This indicates that a change towards a higher growth has occurred in the population of burbots in Sävarån between the years 2001-2014 to 2019.

Figure 7. Length at age of Sävarån between the years 2001-2014 and 2019. Sample size in brackets.

3.2 Size and condition

The mean size differed between sampling periods in Rickleån (table 1). The mean length in both 1969-1971 and 2001-2014 was significantly higher than in 2019 (p<0.05). The weight in 1973 was significantly higher compared to in 2019 (p<0.01). The size did not differ

significantly between 1969-1971 and 2001-2014 in Rickleån. In Sävarån the comparison of mean size did not differ significantly between the sampling periods.

Table 1. Comparison of size in both Rickleån and Sävarån. Year, Sample size (n), min, mean, max, standard deviation (SD) and confidence interval (CI) is presented for the total length (mm) and total weight (g) for Rickleån and Sävarån. 0 100 200 300 400 500 600 700 800 2 3 4 5 6 7 8 9 10

L

en

gt

h

(m

m

)

Age (years)

Length at age - Sävarån

2001-2014

2019

Comparison of size in Rickleån

Total length (mm) Time frame n Min Mean Max SD CI

1969-1971 244 362 518 740 7,07 0,89

2001-2014 263 290 519 685 24,37 2,95

2019 14 280 462 580 87,72 45,95

Total weight (g) Time frame n Min Mean Max SD CI

1969-1971 244 328 987 2870 398,03 49,94 2001-2014 263 135 933 2245 428,53 51,79

2019 14 130 688 1568 414,87 217,32

Comparison of size in Sävarån

Total length (mm) Time frame n Min Mean Max SD CI

2001-2014 357 310 467 695 21,56 2,24

2019 60 260 452 660 71,88 18,19

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In 2019, burbots in Sävarån had a significant higher mean condition-index compared to 2001-2014 (p<0.01) (figure 8). Suggesting an increase in condition of burbots in Sävarån. In Rickleån the mean condition-index did not differ between the sampling years.

Figure 8. The condition-index in both Sävarån and Rickleån between the years 2001-2014 and 2019. Sample size in brackets.

3.3 Liver-index

The mean liver-index per river, sex and year differed significantly between the total of 2001-2014 and 2019 in Sävarån (Figure 9). The mean liver-index in total were significantly lower in 2019 compared to in 2001-2014 in Sävarån (p<0.01). The mean liver-index did not differ between sampling years in Rickleån.

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8

Sävarån

Rickleån

Co

n

di

ti

on

-in

d

ex

Condition-index

2001-2014

2019

Total

Females

Males

Li

ver

-in

d

ex

Liver-index per river, sex and year

Sävarån 2001-2014 Sävarån 2019 Rickleån 2001-2014 Rickleån 2019 P < 0.01 (233, 60, 142, 14) (85, 11, 65, 9) (148, 49, 77, 5) 2001-2014 357 150 625 2285 332,48 34,49 2019 60 101 657 2573 382,51 96,79

Figure 9. The liver-index between the years 2001-2014 and 2019 of both Sävarån and Rickleån. Sample size in brackets.

3.4 Gonad-index

No differences in the mean gonad-index could be found within sex and between years in any of rivers (Figure 10).

Figure 10. Gonad-index per river and sex between the years 2001-2014 and 2019. Sample size in brackets.

4 Discussion

4.1 Sävarån

This study suggests that growth, measured as length at age, in Sävarån has changed over time. The growth has increased between 2001-2014 to 2019. Also, it shows a higher frequency of young individuals and a lack of older ones in 2019 compared to 2001-2014. Furthermore, the condition of burbot caught in 2019 was higher than those caught in 2001-2014. Lastly, the liver-index of burbot caught in 2019 was lower compared to 2001-2014 which may be explained by the increase in growth since Pulliainen and Korhonen (1990) states that all-year-around growth is enabled through metabolization of liver tissue.

Altogether, this study shows that the population of burbot in Sävarån has a higher frequency of young individuals, a higher condition-index and a higher growth rate in 2019 compared to 2001-2014. Thus, the data suggests that an increase in growth and condition in younger burbot have taken place on the same time as older and larger burbot are missing, but can the observed change be explained?

Hofmann and Fischer (2003) have showed that juvenile burbot food uptake and growth increased with higher water temperatures in the littoral zone. The archipelago outside the rivermouth of Sävarån is shallow with sheltered bays and islands which may lead to a rapid increase in water temperature during the summer. The mean air temperature in July during the period 2013-2019 was almost 1 C° higher at the weather station situated at Umeå Airport than during the period 2002-2012 and 0.7 C° higher in September (SMHI 2019). These differences in mean air temperature likely also resulted in a higher water temperature in the archipelago outside of Sävarån. This increase in water temperature may be one reason behind the increase in growth and condition in three ways. First, an increase in water temperature will increased food uptake and growth in burbot (Hofmann and Fischer 2003). However, Elliot (1976) and Jobling (1995) have suggested that if food is a limiting factor, a shift of the maximum growth towards lower temperatures will take place. Still, Elliot (1976) and Jobling

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6

Females

Males

Go

n

ad

-in

d

ex

Gonad-index per river, sex and year

Sävarån 2001-2014 Sävarån 2019 Rickleån 2001-2014 Rickleån 2019 (143, 11, 91, 9) (100, 49, 118, 5)

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(1995) also states that an increase in temperature may lead to an increase in zooplankton biomass which in turn lead to a higher planktivore production and increased food web efficiency. Moreover, Jobling (1995) writes that cyprinids, which are abundant in the archipelago outside of Sävarån, have a high temperature optimum. Altogether this could enhance prey fish abundance for burbot in the archipelago outside Sävarån and hence affect the growth of burbot in Sävarån in a positive way, by an increase in both temperature optimum and food supply.

Secondly, an increase in temperature could have affected the growth of burbot by reducing intraspecific competition. Barron et al (2012), Wolnicki et al. (2002) and Donner and

Eckmann (2011) all have shown that increased water temperatures reduces survival of burbot larvae which in turn can relax negative density dependent effects on growth in turn leading to a larger size at age in adult fish (Walters and Post 1993).

However, the increase in frequency of younger burbots and a lack of older ones do not directly support a decrease in recruitment of burbot in Sävarån, rather it suggests that either an increase in recruitment or an increase in mortality of older burbots have occurred. This suggested increase in recruitment may be explained by the extensive restoration of the whole catchment that began in 2011 (Västerbotten County Administrative Board 2018a). Stapanian et al (2010) states that habitat change is one of the main causes for the worldwide decline in populations of burbots. The restoration seems to have affected the population of salmon in Sävarån in a positive way, with a big increase in both number of juveniles, and an increase in rising salmons since 2011. The suggested increase in recruitment of burbots in Sävarån might also be explained by the liming program that started in the 90’s in Sävarån (Västerbotten County Administrative Board 2018a). Hudd and Kjellman (2002) writes that acidification during hatchment of burbot can cause massive recruitment failures, and since Sävarån suffered from very low pH-values during the 80’s, sometimes below 5.0 during the time of hatching (Västerbotten County Administrative Board 2018a), it might have caused severe recruitment failures in the population of burbot in Sävarån at that time. The burbot might later have experienced a slow recovery, which can be observed at present times.

An increase in mortality of older burbots could be another or an additional cause to the shown increase in frequency of younger burbots and a lack of older ones. This suggested increase could also be due to warmer water temperature. Since temperature preference is lower for older burbots (Hofmann and Fischer 2003) an upper limit of water temperature for older burbots may have been exceeded in the archipelago outside the rivermouth of Sävarån, resulting in a heightened mortality among older burbot.

Even if an increase in frequency of younger burbots and a lack of older ones was shown, it might not be a result of an increase in recruitment or a higher mortality of older burbot. It could also be explained by an unrepresentativeness of the sample taken. Five days of sampling was conducted in Sävarån during weeks 2-4 of 2019. During this period of time, younger burbots might dominate the spots were sampling occurred and thus affecting the representativeness of the sample.

4.2 Rickleån

This study shows that the same thing has happened in the population of burbot in Rickleån as in the population of burbot in Sävarån, in that growth measured as length at age has changed over time. At the age of 3 the population of burbot in Rickleån was significantly longer than in both 1969-1971 and 2001-2014, indicating an increase in growth of young burbots in Rickleån. From the age of 4 to 9 the growth in the population of burbot in Rickleån has declined from 1969-1971 to 2001-2014, suggesting a decrease in growth of older burbots. Moreover, the mean length of burbots caught in 2019 was significantly lower compared to both 1969-1971 and 2001-2014, and also the mean weight was significantly lower in 2019 compared to 1969-1971. This decrease in size might be due to the low sample size of 14 burbots in 2019 which creates a higher risk of unrepresentativeness of the sample taken. The

low sample size could also explain the change towards a higher frequency of younger burbots and a lack of older ones. But what could be the cause behind the increase in growth of young burbots and the decrease in older?

Food supply and temperature are as mentioned one of the two most important variables determining growth of fishes (Hofmann and Fischer 2002). Therefore, a change in either of these variables seems to be a reasonable explanation for the change in growth. The bay outside the rivermouth of Rickleån is open and exposed, and the islands are few. This should lead to a slower increase in the water temperature than the archipelago outside of Sävarån and also make it a poorer environment. But since the mean air temperature during the summer in Sweden has gone up by 1 C° from the time frame 1960-1972 compared to 1999-2014 (SMHI 2019b) it seems reasonable to think that the water temperature during summer in the bay outside of Rickleån also have increased over the years. Lefébure (2012) states that higher water temperature lead to a higher metabolic rate in fish and thereby increasing their food demand required to maintain a constant body weight. Moreover, the author writes that at a low food supply it is more beneficial to be at low temperatures, where metabolism is low. An assumption that can be made based on this information is that the water temperature in the bay outside of Rickleån has increased, but due to the more poor environment than the one found in the archipelago of Sävarån, the food supply has not increased enough for the older burbots in Rickleån to maintain the growth found in 1969-1971. However, since an increase in growth is shown in younger burbots, a difference in diet between younger and older burbot might be one explanation for the shown dissimilarities.

4.3 Conclusions

A shift towards warmer water temperatures might be an explanation in the observed change in both growth and condition in the population of burbots in Sävarån and Rickleån. The observed change in age frequency in Sävarån suggests that either an increase in recruitment or an increase in mortality of older burbots have occurred. This could be a result of an

increase in water temperature but could also be an effect of restoration and the control of pH in Sävarån. The predicted rise in water temperature in the Baltic Sea due to climate change will have large impacts on its ecology (HELCOM 2013). This study indicates that an increase in water temperature in the Baltic Sea might impact the population of Burbot and its ecology. Further studies on how this observed change in growth, condition and age frequency in the population of burbot in the Baltic Sea might impact species composition, food webs and more are needed.

5 Acknowledgements

First and foremost, I would like to thank my supervisor Pär Byström for all the help, guidance and input along the way. Furthermore, I wish to express my gratefulness to my wife for all the support, help and encouragement. Also, I wish not to forget the people who helped me with sampling and kept me company during the cold winter nights, like Björn Sundqvist, Rasmus Domanders, Fredrik Grafström and more.

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