Suffolk Bancorp, parent of the 27-branch Suffolk County National Bank, said Thursday its fourth quarter net income fell by 10.6 percent from a year earlier, mostly on the costs of a planned computer systems conversion.

The Riverhead-based company said net income was $3.64 million or 31 cents a share for the three months ended Dec. 31, down from $4.07 million, or 35 cents a share a year earlier.

Without the systems conversion costs, net income in the quarter would have been $4.7 million or 40 cents a share. The costs totaled $1.4 million, largely for payments to terminate existing contracts with the current provider of the bank’s core operating system and with other vendors and service providers, the bank said.

“Like many community banks, we have, over many decades, layered numerous products and services on top of our core system, creating a complex technology environment that is slow, difficult to manage, and has made introducing new products and services challenging and inefficient,” president and chief executive Howard C. Bluver said in a statement.

The conversion will done in this year’s second quarter, said chief financial officer Brian K. Finneran.

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Suffolk also cited as a factor in the earnings drop a higher effective federal, state and New York City income tax rate in the fourth quarter from a year earlier. Suffolk has a branch in Queens.

For the year, Suffolk’s net income was $17.7 million, or $1.49 a share, versus $15.3 million or $1.31 a share for 2014.

Total assets at year-end were $2.2 billion, up 14 percent from a year earlier.

Suffolk’s shares were trading at $26.89, up $1.40 at 3:17 p.m. yesterday on the New York Stock Exchange.