online. Within hunting, our firearm category saw an increase of $19.6 million or 28.4% and our ammunition category saw a decrease of $3.8 million or 6.5%, in the first quarter of fiscal year 2021 compared to the comparable 13-week period of fiscal year 2020. The decrease seen in the ammunition category is due to the lack of supply in the market as a result of supply chain disruptions caused by the COVID-19 pandemid and demand that is outpacing manufacturing capacity.
Each of our departments also had increases in same store sales, for the 13 weeks ended May 1, 2021. Our clothing, footwear, optics, electronics, and accessories, fishing, camping, and hunting departments had increases of 85.4%, 73.0%, 46.7%, 40.3%, 39.1% and 9.6%, respectively, for the first quarter of fiscal year 2021 compared to the comparable 13-week period of fiscal year 2020. We saw a substantial increase in same store sales for our clothing and footwear departments because of a significant change in consumer behavior as it relates to the purchasing of clothing and footwear in during the first quarter of 2021 compared to the first quarter of 2020. As of May 1, 2021, we had 105 stores included in our same store sales calculation.
Gross Profit. Gross profit increased to $104.0 million during the 13 weeks ended May 1, 2021 compared to $74.8 million for the corresponding period of fiscal year 2020 primarily as a result of increased sales. As a percentage of net sales, gross profit increased to 31.8% for the 13 weeks ended May 1, 2021, compared to 30.3% for the corresponding period of fiscal year 2020 due to a higher portion of our sales coming from our clothing and footwear categories, which tend to have better margins. Gross margin also increased due to volume incentives, and other adjustments. We expect a continued higher than normal proportion of revenue to come from firearms and ammunition, and a higher volume of sales to be conducted through our e-commerce platform. Both of these factors will continue to put downward pressure on gross margin which may be partially offset by improved product margins and increased sales in higher margin categories such as clothing and footwear.
Selling, General, and Administrative Expenses. Selling, general, and administrative expenses increased by $15.2 million, or 20.2%, to $90.4 million during the 13 weeks ended May 1, 2021 from $75.2 million for the comparable 13-week period of fiscal year 2020. This increase was primarily due to an increase in our payroll expense of $10.2 million, which was the result of opening 7 new stores since the end of the first quarter of fiscal year 2020, and minimum wage increases impacting 12 of our stores in fiscal year 2021. We also had increases in acquisition costs of $2.8 million due to costs relating to the pending merger with Great Outdoors Group, as well as increases in rent and other selling, general and administrative costs of $1.8 million, and $0.9 million, respectively, during the 13 weeks ended May 1, 2021 primarily related to the opening of 7 new stores since May 2, 2020. These increases were partially offset by a reduction in depreciation and pre-opening expenses of $0.4 million and $0.2 million, respectively, during the 13 weeks ended May 1, 2021. As a percentage of net sales, selling, general, and administrative expenses decreased to 27.8% of net sales in the first quarter of fiscal year 2021, compared to 30.5% of net sales in the first quarter of fiscal year 2020, primarily as a result of the significant increase in net sales we experienced in the first quarter of fiscal year 2021 compared to the first quarter of fiscal year 2020.
Interest Expense. Interest expense decreased by $1.3 million, or 85.3%, to $0.2 million during the 13 weeks ended May 1, 2021 from $1.5 million for the comparable 13-week period of fiscal year 2020. Interest expense decreased primarily as a result of no debt balance during the first quarter of fiscal year 2021 compared to the first quarter of fiscal year 2020. The interest expense recorded during the 13 weeks ended May 1, 2021 consists of fees charged on the unused portion of our line of credit.
Income Taxes. We recognized income tax expense of $3.0 million compared to an income tax benefit of $0.8 million during the 13 weeks ended May 1, 2021 and May 2, 2020, respectively. Our effective tax rate for the 13 weeks ended May 1, 2021 and May 2, 2020 was 22.0% and 42.9%, respectively. Our effective tax rate will generally differ from the U.S. Federal statutory rate of 21.0%, due to state taxes, permanent items, and discrete items relating to stock award deductions.
Seasonality
Due to the openings of hunting season across the country and consumers’ holiday buying patterns, net sales are typically higher in the third and fourth fiscal quarters than in the first and second fiscal quarters. We also incur additional expenses in the third and fourth fiscal quarters due to higher sales volume and increased staffing in our stores. While we typically would expect our net sales to continue to reflect this seasonal pattern, we may not recognize higher sales